In this paper, the patterns of variability in the use of auxiliary verbs in English as a second language from speakers with a Persian ethnic background are compared to those of Persian, and English. The findings of the comparisons between L1, L2, and English help us determine how variable L2 is and what the sources of this variation are. Our results indicate that the factors conditioning the variable contraction of auxiliaries in advanced L2 strongly parallel the effects observed in the native English speech. These results argue against transfer from L1 or independent creation by the speakers (Interlanguage). Rather, they suggest that the constraints on variable features are as much a part of acquisition as the features themselves. (Contains 4 figures and 5 tables.)