Purpose: Prior work (Guo, Owen, & Tomblin, 2010) has shown that at the group level, auxiliary "is" production by young English-speaking children was symmetrical across lexical noun and pronominal subjects. Individual data did not uniformly reflect these patterns. On the basis of the framework of the gradual morphosyntactic learning (GML) hypothesis, the authors tested whether the addition of a theoretically motivated developmental measure, tense productivity (TP), could assist in explaining these individual differences. Method: Using archival data from 20 children between age 2;8 and 3;4 (years;months), the authors tested the ability of 3 developmental measures (TP; finite verb morphology composite, FVMC; mean length of utterance, MLU) to predict use of auxiliary "is" with different subject types. Results: TP, but not MLU or FVMC, significantly improved model fit. Children with low TP scores produced auxiliary "is" more accurately with pronominal subjects than with lexical subjects. The facilitative effect of pronominal subjects on the production of auxiliary "is," however, was not found in children with high TP scores. Conclusion: The finding that the effect of subject types on the production accuracy of auxiliary "is" changed with children's TP is consistent with the GML hypothesis.