diff --git a/internal/gps/satisfy.go b/internal/gps/satisfy.go
index 9638d766ff..c7031e65e7 100644
--- a/internal/gps/satisfy.go
+++ b/internal/gps/satisfy.go
@@ -54,6 +54,9 @@ func (s *solver) check(a atomWithPackages, pkgonly bool) error {
 		if err = s.checkIdentMatches(a, dep); err != nil {
 			return err
 		}
+		if err = s.checkCaseConflicts(a, dep); err != nil {
+			return err
+		}
 		if err = s.checkDepsConstraintsAllowable(a, dep); err != nil {
 			return err
 		}
@@ -218,6 +221,31 @@ func (s *solver) checkIdentMatches(a atomWithPackages, cdep completeDep) error {
 	return nil
 }
 
+// checkCaseConflicts ensures that the ProjectRoot specified in the completeDep
+// does not have case conflicts with any existing dependencies.
+//
+// We only need to check the ProjectRoot, rather than any packages therein, as
+// the later check for package existence is case-sensitive.
+func (s *solver) checkCaseConflicts(a atomWithPackages, cdep completeDep) error {
+	pr := cdep.workingConstraint.Ident.ProjectRoot
+	hasConflict, current := s.sel.findCaseConflicts(pr)
+	if !hasConflict {
+		return nil
+	}
+
+	curid, _ := s.sel.getIdentFor(pr)
+	deps := s.sel.getDependenciesOn(curid)
+	for _, d := range deps {
+		s.fail(d.depender.id)
+	}
+
+	return &caseMismatchFailure{
+		goal:    dependency{depender: a.a, dep: cdep},
+		current: current,
+		failsib: deps,
+	}
+}
+
 // checkPackageImportsFromDepExist ensures that, if the dep is already selected,
 // the newly-required set of packages being placed on it exist and are valid.
 func (s *solver) checkPackageImportsFromDepExist(a atomWithPackages, cdep completeDep) error {
diff --git a/internal/gps/selection.go b/internal/gps/selection.go
index e29d83fe53..02ea1f2d5b 100644
--- a/internal/gps/selection.go
+++ b/internal/gps/selection.go
@@ -4,9 +4,12 @@
 
 package gps
 
+import "strings"
+
 type selection struct {
 	projects []selected
 	deps     map[ProjectRoot][]dependency
+	prLenMap map[int][]ProjectRoot
 	vu       *versionUnifier
 }
 
@@ -59,13 +62,43 @@ func (s *selection) popSelection() (atomWithPackages, bool) {
 	return sel.a, sel.first
 }
 
+func (s *selection) findCaseConflicts(pr ProjectRoot) (bool, ProjectRoot) {
+	prlist, has := s.prLenMap[len(pr)]
+	if !has {
+		return false, ""
+	}
+
+	// TODO(sdboyer) bug here if it's possible that strings.ToLower() could
+	// change the length of the string
+	lowpr := strings.ToLower(string(pr))
+	for _, existing := range prlist {
+		if lowpr != strings.ToLower(string(existing)) {
+			continue
+		}
+		// If the converted strings match, then whatever we figure out here will
+		// be definitive - we needn't walk the rest of the slice.
+		if pr == existing {
+			return false, ""
+		} else {
+			return true, existing
+		}
+	}
+
+	return false, ""
+}
+
 func (s *selection) pushDep(dep dependency) {
-	s.deps[dep.dep.Ident.ProjectRoot] = append(s.deps[dep.dep.Ident.ProjectRoot], dep)
+	pr := dep.dep.Ident.ProjectRoot
+	s.deps[pr] = append(s.deps[pr], dep)
+	s.prLenMap[len(pr)] = append(s.prLenMap[len(pr)], pr)
 }
 
 func (s *selection) popDep(id ProjectIdentifier) (dep dependency) {
 	deps := s.deps[id.ProjectRoot]
 	dep, s.deps[id.ProjectRoot] = deps[len(deps)-1], deps[:len(deps)-1]
+
+	prlist := s.prLenMap[len(id.ProjectRoot)]
+	s.prLenMap[len(id.ProjectRoot)] = prlist[:len(prlist)-1]
 	return dep
 }
 
diff --git a/internal/gps/solve_failures.go b/internal/gps/solve_failures.go
index e6a2c47a85..b3779756cc 100644
--- a/internal/gps/solve_failures.go
+++ b/internal/gps/solve_failures.go
@@ -71,6 +71,49 @@ func (e *noVersionError) traceString() string {
 	return buf.String()
 }
 
+// caseMismatcFailure occurs when there are import paths that differ only by
+// case. The compiler disallows this case.
+type caseMismatchFailure struct {
+	// goal is the depender atom that tried to introduce the case-varying name,
+	// along with the case-varying name.
+	goal dependency
+	// current is the specific casing of a ProjectRoot that is presently
+	// selected for all possible case variations of its contained unicode code
+	// points.
+	current ProjectRoot
+	// failsib is the list of active dependencies that have determined the
+	// specific casing for the target project.
+	failsib []dependency
+}
+
+func (e *caseMismatchFailure) Error() string {
+	if len(e.failsib) == 1 {
+		str := "Could not introduce %s due to a case-only variation: it depends on %q, but %q was already established as the case variant for that project root by depender %s"
+		return fmt.Sprintf(str, a2vs(e.goal.depender), e.goal.dep.Ident.ProjectRoot, e.current, a2vs(e.failsib[0].depender))
+	}
+
+	var buf bytes.Buffer
+
+	str := "Could not introduce %s due to a case-only variation: it depends on %q, but %q was already established as the case variant for that project root by the following other dependers:\n"
+	fmt.Fprintf(&buf, str, e.goal.dep.Ident.ProjectRoot, e.current, a2vs(e.goal.depender))
+
+	for _, c := range e.failsib {
+		fmt.Fprintf(&buf, "\t%s\n", a2vs(c.depender))
+	}
+
+	return buf.String()
+}
+
+func (e *caseMismatchFailure) traceString() string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "case-only variation in dependency on %q; %q already established by:\n", e.goal.dep.Ident.ProjectRoot, e.current)
+	for _, f := range e.failsib {
+		fmt.Fprintf(&buf, "%s\n", a2vs(f.depender))
+	}
+
+	return buf.String()
+}
+
 // disjointConstraintFailure occurs when attempting to introduce an atom that
 // itself has an acceptable version, but one of its dependency constraints is
 // disjoint with one or more dependency constraints already active for that
diff --git a/internal/gps/solver.go b/internal/gps/solver.go
index 0515f199dd..9e91665b29 100644
--- a/internal/gps/solver.go
+++ b/internal/gps/solver.go
@@ -307,8 +307,9 @@ func Prepare(params SolveParameters, sm SourceManager) (Solver, error) {
 
 	// Initialize stacks and queues
 	s.sel = &selection{
-		deps: make(map[ProjectRoot][]dependency),
-		vu:   s.vUnify,
+		deps:     make(map[ProjectRoot][]dependency),
+		prLenMap: make(map[int][]ProjectRoot),
+		vu:       s.vUnify,
 	}
 	s.unsel = &unselected{
 		sl:  make([]bimodalIdentifier, 0),