diff --git a/setup/bootstrap.sh b/setup/bootstrap.sh
index 305a06e..f46e5f1 100644
--- a/setup/bootstrap.sh
+++ b/setup/bootstrap.sh
@@ -7,7 +7,34 @@
 #########################################################
 
 if [ -z "$TAG" ]; then
-	TAG=v0.30
+	# If a version to install isn't explicitly given as an environment
+	# variable, then install the latest version. But the latest version
+	# depends on the operating system. Existing Ubuntu 14.04 users need
+	# to be able to upgrade to the latest version supporting Ubuntu 14.04,
+	# in part because an upgrade is required before jumping to Ubuntu 18.04.
+	# New users on Ubuntu 18 need to get the latest version number too.
+	#
+	# Also, the system status checks read this script for TAG= to get
+	# the latest version, so the first TAG= line must be the one that
+	# we want to display in status checks.
+	if [ "`lsb_release -d | sed 's/.*:\s*//' | sed 's/18\.04\.[0-9]/18.04/' `" == "Ubuntu 18.04 LTS" ]; then
+		# This machine is running Ubuntu 18.04.
+		TAG=v0.30
+
+	elif [ "`lsb_release -d | sed 's/.*:\s*//' | sed 's/14\.04\.[0-9]/14.04/' `" == "Ubuntu 14.04 LTS" ]; then
+		# This machine is running Ubuntu 14.04.
+		echo "You are installing the last version of Mail-in-a-Box that will"
+		echo "support Ubuntu 14.04. If this is a new installation of Mail-in-a-Box,"
+		echo "stop now and switch to a machine running Ubuntu 18.04. If you are"
+		echo "upgrading an existing Mail-in-a-Box --- great. After upgrading this"
+		echo "box, please visit https://mailinabox.email for notes on how to upgrade"
+		echo "to Ubuntu 18.04."
+		TAG=v0.30
+
+	else
+		echo "This script must be run on a system running Ubuntu 18.04 or Ubuntu 14.04."
+		exit
+	fi
 fi
 
 # Are we running as root?