You need to pass in the binding of the current method.
@bubbles = Bubble.find(:all, :limit => 30) # What is the limit? open( "public/xml_data/main_data_feed.xml", "w" ) { | l | l.write ERB.new( IO.read( File.join( RAILS_ROOT, "app/views/rssfeed/bubbles.rhtml" ) ) ).result(binding) }