diff --git a/json.go b/json_parser.go
similarity index 100%
rename from json.go
rename to json_parser.go
diff --git a/json_test.go b/json_parser_test.go
similarity index 100%
rename from json_test.go
rename to json_parser_test.go
diff --git a/options.go b/options.go
deleted file mode 100644
index de09749..0000000
--- a/options.go
+++ /dev/null
@@ -1 +0,0 @@
-package ff
diff --git a/parse.go b/parse.go
index 0c53daf..a73c74a 100644
--- a/parse.go
+++ b/parse.go
@@ -1,9 +1,12 @@
 package ff
 
 import (
+	"embed"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
+	iofs "io/fs"
 	"os"
 	"strings"
 )
@@ -12,8 +15,6 @@ import (
 // and calls the set function for each parsed flag pair.
 type ConfigFileParser func(r io.Reader, set func(name, value string) error) error
 
-type lookupFunc func(fs *flag.FlagSet, name string) *flag.Flag
-
 // Parse the flags in the flag set from the provided (presumably commandline)
 // args. Additional options may be provided to have Parse also read from a
 // config file, and/or environment variables, in that priority order.
@@ -97,9 +98,9 @@ func Parse(fs *flag.FlagSet, args []string, options ...Option) error {
 		}
 	}
 
-	if c.configFileLookup == nil {
-		c.configFileLookup = func(fs *flag.FlagSet, name string) *flag.Flag {
-			return fs.Lookup(name)
+	if c.configFileOpenFunc == nil {
+		c.configFileOpenFunc = func(s string) (iofs.File, error) {
+			return os.Open(s)
 		}
 	}
 
@@ -109,7 +110,7 @@ func Parse(fs *flag.FlagSet, args []string, options ...Option) error {
 		parseConfigFile = haveConfigFile && haveParser
 	)
 	if parseConfigFile {
-		f, err := os.Open(configFile)
+		f, err := c.configFileOpenFunc(configFile)
 		switch {
 		case err == nil:
 			defer f.Close()
@@ -151,7 +152,7 @@ func Parse(fs *flag.FlagSet, args []string, options ...Option) error {
 				return err
 			}
 
-		case os.IsNotExist(err) && c.allowMissingConfigFile:
+		case errors.Is(err, iofs.ErrNotExist) && c.allowMissingConfigFile:
 			// no problem
 
 		default:
@@ -171,7 +172,7 @@ type Context struct {
 	configFileVia          *string
 	configFileFlagName     string
 	configFileParser       ConfigFileParser
-	configFileLookup       lookupFunc
+	configFileOpenFunc     func(string) (iofs.File, error)
 	allowMissingConfigFile bool
 	readEnvVars            bool
 	envVarPrefix           string
@@ -278,6 +279,15 @@ func WithIgnoreUndefined(ignore bool) Option {
 	}
 }
 
+// WithFilesystem tells Parse to use the provided filesystem when accessing
+// files on disk, for example when reading a config file. By default, the host
+// filesystem is used, via [os.Open].
+func WithFilesystem(fs embed.FS) Option {
+	return func(c *Context) {
+		c.configFileOpenFunc = fs.Open
+	}
+}
+
 var flagNameToEnvVar = strings.NewReplacer(
 	"-", "_",
 	".", "_",
diff --git a/parse_test.go b/parse_test.go
index 9717f7b..b90420d 100644
--- a/parse_test.go
+++ b/parse_test.go
@@ -2,6 +2,7 @@ package ff_test
 
 import (
 	"context"
+	"embed"
 	"flag"
 	"os"
 	"testing"
@@ -12,6 +13,9 @@ import (
 	"github.com/peterbourgon/ff/v3/fftest"
 )
 
+//go:embed testdata/*.conf
+var testdataConfigFS embed.FS
+
 func TestParseBasics(t *testing.T) {
 	t.Parallel()
 
@@ -149,6 +153,11 @@ func TestParseBasics(t *testing.T) {
 			opts: []ff.Option{ff.WithEnvVarNoPrefix()},
 			want: fftest.Vars{S: "xxx", F: 9.87},
 		},
+		{
+			name: "WithFilesystem testdata/1.conf",
+			opts: []ff.Option{ff.WithFilesystem(testdataConfigFS), ff.WithConfigFile("testdata/1.conf"), ff.WithConfigFileParser(ff.PlainParser)},
+			want: fftest.Vars{S: "bar", I: 99, B: true, D: 1 * time.Hour},
+		},
 	} {
 		t.Run(testcase.name, func(t *testing.T) {
 			if testcase.file != "" {