diff --git a/go/analysis/passes/printf/printf.go b/go/analysis/passes/printf/printf.go
index c0265aa..b73ff5e 100644
--- a/go/analysis/passes/printf/printf.go
+++ b/go/analysis/passes/printf/printf.go
@@ -453,15 +453,23 @@
 }
 
 // isFormatter reports whether t satisfies fmt.Formatter.
-// Unlike fmt.Stringer, it's impossible to satisfy fmt.Formatter without importing fmt.
-func isFormatter(pass *analysis.Pass, t types.Type) bool {
-	for _, imp := range pass.Pkg.Imports() {
-		if imp.Path() == "fmt" {
-			formatter := imp.Scope().Lookup("Formatter").Type().Underlying().(*types.Interface)
-			return types.Implements(t, formatter)
-		}
+// The only interface method to look for is "Format(State, rune)".
+func isFormatter(typ types.Type) bool {
+	obj, _, _ := types.LookupFieldOrMethod(typ, false, nil, "Format")
+	fn, ok := obj.(*types.Func)
+	if !ok {
+		return false
 	}
-	return false
+	sig := fn.Type().(*types.Signature)
+	return sig.Params().Len() == 2 &&
+		sig.Results().Len() == 0 &&
+		isNamed(sig.Params().At(0).Type(), "fmt", "State") &&
+		types.Identical(sig.Params().At(1).Type(), types.Typ[types.Rune])
+}
+
+func isNamed(T types.Type, pkgpath, name string) bool {
+	named, ok := T.(*types.Named)
+	return ok && named.Obj().Pkg().Path() == pkgpath && named.Obj().Name() == name
 }
 
 // formatState holds the parsed representation of a printf directive such as "%3.*[4]d".
@@ -753,7 +761,7 @@
 	formatter := false
 	if state.argNum < len(call.Args) {
 		if tv, ok := pass.TypesInfo.Types[call.Args[state.argNum]]; ok {
-			formatter = isFormatter(pass, tv.Type)
+			formatter = isFormatter(tv.Type)
 		}
 	}
 
@@ -831,7 +839,7 @@
 	typ := pass.TypesInfo.Types[e].Type
 
 	// It's unlikely to be a recursive stringer if it has a Format method.
-	if isFormatter(pass, typ) {
+	if isFormatter(typ) {
 		return false
 	}
 
diff --git a/go/analysis/passes/printf/printf_test.go b/go/analysis/passes/printf/printf_test.go
index 937bbe7..6845326 100644
--- a/go/analysis/passes/printf/printf_test.go
+++ b/go/analysis/passes/printf/printf_test.go
@@ -10,5 +10,5 @@
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
 	printf.Analyzer.Flags.Set("funcs", "Warn,Warnf")
-	analysistest.Run(t, testdata, printf.Analyzer, "a", "b")
+	analysistest.Run(t, testdata, printf.Analyzer, "a", "b", "nofmt")
 }
diff --git a/go/analysis/passes/printf/testdata/src/nofmt/nofmt.go b/go/analysis/passes/printf/testdata/src/nofmt/nofmt.go
new file mode 100644
index 0000000..c323796
--- /dev/null
+++ b/go/analysis/passes/printf/testdata/src/nofmt/nofmt.go
@@ -0,0 +1,10 @@
+package b
+
+import (
+	"math/big"
+	"testing"
+)
+
+func formatBigInt(t *testing.T) {
+	t.Logf("%d\n", big.NewInt(4))
+}
diff --git a/go/analysis/passes/printf/types.go b/go/analysis/passes/printf/types.go
index 2eb6dc7..12286fd 100644
--- a/go/analysis/passes/printf/types.go
+++ b/go/analysis/passes/printf/types.go
@@ -38,7 +38,7 @@
 		}
 	}
 	// If the type implements fmt.Formatter, we have nothing to check.
-	if isFormatter(pass, typ) {
+	if isFormatter(typ) {
 		return true
 	}
 	// If we can use a string, might arg (dynamically) implement the Stringer or Error interface?