BASIC 3.5 is an extension released by 64'er Magazine back the June 1990 Issue that allows running Commodore BASIC v3.5 programs on a Commodore 64.

While doing some searching on this extension, I ran across an interesting post on StackExchange that describes a couple ways to autoload/wedge you programs so they load the extension automatically and continue to run.  Here's a link for the curious.