diff --git a/Makefile.am b/Makefile.am
index 5a3a1a69..37198690 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,5 +1,9 @@
 
-SUBDIRS = gdbus gatchat gisi include plugins drivers unit src doc
+SUBDIRS = gdbus gatchat include plugins drivers unit src doc
+
+if WANT_ISI
+  SUBDIRS += gisi
+endif
 
 AM_MAKEFLAGS = --no-print-directory
 
diff --git a/configure.ac b/configure.ac
index c57811cf..796aa21b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -48,6 +48,11 @@ AC_ARG_ENABLE(pie, AC_HELP_STRING([--enable-pie],
 AC_ARG_ENABLE(threads, AC_HELP_STRING([--enable-threads],
 		[enable threading support]), [enable_threads=${enableval}])
 
+AC_ARG_ENABLE(isi, AC_HELP_STRING([--disable-isi],
+		[disable PhoNet/ISI protocol support]), [enable_isi=${enableval}])
+
+AM_CONDITIONAL(WANT_ISI, test "${enable_isi}" != "no")
+
 AC_CHECK_LIB(dl, dlopen, dummy=yes,
 			AC_MSG_ERROR(dynamic linking loader is required))
 
diff --git a/drivers/Makefile.am b/drivers/Makefile.am
index 28fde60a..30d547ff 100644
--- a/drivers/Makefile.am
+++ b/drivers/Makefile.am
@@ -13,8 +13,10 @@ builtin_sources += atmodem/atmodem.c atmodem/at.h \
 			atmodem/call-barring.c atmodem/phonebook.c \
 			atmodem/ssn.c
 
-builtin_modules += isimodem
-builtin_sources += isimodem/isimodem.c isimodem/isi.h isimodem/isiphonebook.c
+if WANT_ISI
+  builtin_modules += isimodem
+  builtin_sources += isimodem/isimodem.c isimodem/isi.h isimodem/isiphonebook.c
+endif
 
 noinst_LTLIBRARIES = libbuiltin.la