One branch of research in second language acquisition has investigated the ways a learner's interlanguage (IL) varies between tasks. IL variation research has examined linguistic, psycholinguistic, and sociolinguistic constraints, and has revealed much about this phenomenon. An additional potentially-useful perspective that has, to this point, been virtually unused in IL variation research and theory, is the cognitive psychology construct transfer of learning. This paper examines the relationship between IL variation and transfer of learning. An argument is made that IL performance is learning transfer; so, variation in IL performance may in some cases be related to constraints on learning transfer. If that is the case, research and theory on learning transfer can contribute to research and theory on IL variation, and several ways this may happen are described. (Contains 2 tables, 1 figure and 10 notes.)