From the Ursack FAQ:
"Is Ursack resistant to rodents, marmots, wolves, etc?
Ursack is highly effective against Marmots and raccoons. It should not be considered wolf or dog resistant. That's because they have sharp teeth and strong jaws. Bears have great strength, but their teeth are not sharp. We now make a rodent resistant bag, the Ursack Minor. It is not effective against bears although we think it is more effective against rodents than the AllWhite. Other rodents have varying degrees of success. Sometimes mice can chew very small holes, but very little of your food is likely to be taken. Some species of squirrels and (we have heard) pine martens have greater success, and may on rare occasion chew holes in bear resistant Ursacks."