The role of lexical frequency in language variation and change has received considerable attention in recent years. Recently Erker and Guy (2012) extended the analysis of frequency effects to morphosyntactic variation. Based on data from 12 Dominican and Mexican speakers from Otheguy and Zentella’s (2012) New York City Spanish corpus, they examined the role of frequency in variation between null and overt subject personal pronouns (SPP). Their results suggest that frequency either activates or amplifies the effects of other constraints such as co-reference. This paper attempts to replicate Erker and Guy’s study with a data set of Mexican immigrant and Mexican American Spanish. Analysis of more than 8,600 tokens shows that frequency has only a small effect on SPP use. In separate analyses of frequent and non-frequent verb forms, fewer constraints reach significance with frequent verb forms only than with non-frequent forms only. Moreover, in cases where constraints reach significance in both analyses, effects are stronger with non-frequent than with frequent forms. Finally, when all verb forms are combined in a single analysis, non-frequent forms are significantly more likely than frequent forms to co-occur with overt SPPs. We conclude that claims about frequency effects in SPP variation should be treated with caution and that further analyses are needed to establish whether models incorporating frequency can be extended to this area of the grammar.