Purpose: In this study, the authors tested the unique checking constraint (UCC) hypothesis and the usage-based approach concerning why young children variably use tense and agreement morphemes in obligatory contexts by examining the effect of subject types on the production of auxiliary "is". Method: Twenty typically developing 3-year-olds were included in this study. The children's production of auxiliary "is" was elicited in sentences with pronominal subjects, high-frequency lexical noun phrase (NP) subjects (e.g., "the dog"), and low-frequency lexical NP subjects (e.g., "the deer"). Results: As a group, children did not use auxiliary "is" more accurately with pronominal subjects than with lexical NP subjects. Furthermore, individual data revealed that although some children used auxiliary "is" more accurately with pronominal subjects than with lexical NP subjects, the majority of children did not show this trend. Conclusion: The symmetry observed between lexical and pronominal subjects supports the predictions of the UCC hypothesis, although additional mechanisms may be needed to account for the asymmetry between subject types in some individual children. Discrepant results between the present study and previous studies were attributed to differences in task formats and children's developmental levels.