[flexcoders] odd script error - undefined property

[jmerrill_2001]

my entire MXML & Actionscript, which I can't see any errors with:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"; 
layout="absolute">
        <mx:Script>
                <![CDATA[
                        public var foo:String;
                        foo = "Hello World";
                ]]>
        </mx:Script>
</mx:Application>

I am getting this error on that second line in the script: 
"1120: Access of undefined property foo."

??? Any ideas?  Haven't I already defined the variable "foo"???

Thanks,

Jason