diff --git a/util/resolve/api.go b/util/resolve/api.go
index 7d2ff21..c4aa722 100644
--- a/util/resolve/api.go
+++ b/util/resolve/api.go
@@ -144,6 +144,10 @@ func (a *APIClient) Versions(ctx context.Context, pk PackageKey) ([]Version, err
 			Version:     v.VersionKey.Version,
 		}, v, "")
 	}
+	// Sort the versions into the ascending order expected by the resolvers;
+	// the API may return them in a different order (e.g. lexicographic for
+	// PyPI).
+	SortVersions(vers)
 	return vers, nil
 }
 
diff --git a/util/resolve/api_test.go b/util/resolve/api_test.go
index f9565fe..0d17c7c 100644
--- a/util/resolve/api_test.go
+++ b/util/resolve/api_test.go
@@ -19,6 +19,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"google.golang.org/grpc"
 
 	pb "deps.dev/api/v3"
 	"deps.dev/util/resolve/internal/deptest"
@@ -326,3 +327,58 @@ func TestPyPIRequirements(t *testing.T) {
 		}
 	}
 }
+
+// mockInsightsClient is a pb.InsightsClient that returns a fixed package
+// from GetPackage; the other methods are not implemented and are not
+// called by the tests.
+type mockInsightsClient struct {
+	pb.InsightsClient
+	pkg *pb.Package
+}
+
+func (m *mockInsightsClient) GetPackage(ctx context.Context, in *pb.GetPackageRequest, opts ...grpc.CallOption) (*pb.Package, error) {
+	return m.pkg, nil
+}
+
+func TestVersions(t *testing.T) {
+	ctx := context.Background()
+	pk := PackageKey{
+		System: PyPI,
+		Name:   "test",
+	}
+	vers := func(v string) *pb.Package_Version {
+		return &pb.Package_Version{
+			VersionKey: &pb.VersionKey{
+				System:  pb.System_PYPI,
+				Name:    "test",
+				Version: v,
+			},
+		}
+	}
+	// The versions in the lexicographic order returned by the deps.dev API
+	// for PyPI, where "9.1.0" comes after "84.0.0".
+	client := APIClient{
+		c: &mockInsightsClient{pkg: &pb.Package{
+			Versions: []*pb.Package_Version{
+				vers("0.9.0"),
+				vers("1.10.0"),
+				vers("1.9.0"),
+				vers("2.0.0"),
+				vers("84.0.0"),
+				vers("9.1.0"),
+			},
+		}},
+	}
+	got, err := client.Versions(ctx, pk)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var gotVersions []string
+	for _, v := range got {
+		gotVersions = append(gotVersions, v.Version)
+	}
+	wantVersions := []string{"0.9.0", "1.9.0", "1.10.0", "2.0.0", "9.1.0", "84.0.0"}
+	if d := cmp.Diff(wantVersions, gotVersions); d != "" {
+		t.Errorf("Versions(%v):\n(- want, + got):\n%s", pk, d)
+	}
+}
diff --git a/util/resolve/client.go b/util/resolve/client.go
index 8448579..a5fb4fd 100644
--- a/util/resolve/client.go
+++ b/util/resolve/client.go
@@ -46,7 +46,8 @@ type Client interface {
 	// Version finds a particular version, providing access to its
 	// attributes.
 	Version(context.Context, VersionKey) (Version, error)
-	// Versions returns all the known versions of a package.
+	// Versions returns all the known versions of a package, in a
+	// system-specific order, expected by the relevant resolver.
 	Versions(context.Context, PackageKey) ([]Version, error)
 	// Requirements returns the direct dependencies of the provided version.
 	Requirements(context.Context, VersionKey) ([]RequirementVersion, error)