I've been reading up on this. I bought yeast from costco on 8/13/11 and over the weekend had to throw it out because it lost all of its muster/strength after my dough failed to rise on several bakes. There was like over half left. I kept in a tupperware on the bottom shelf in my frig. And everytime I open it up condensation would enter the yeast, lessening it's strength. I've read where some people just keep it in a baggie in the freezer and just take out what you need for your bakes.