This study examines the relationship between third person singular (3Psg) subject pronoun case and agreement, focusing on the hypothesis that these two grammatical subsystems develop together. This hypothesis is broken down into two separate, empirically testable hypotheses: (a) that correct subject case pronoun production and the production of agreement are correlated, and (b) that at the sentence level, correct case is dependent on the presence of agreement. Twenty-nine children between the ages of 2;6 and 4;0 were each audiotaped for approximately two hours playing and interacting with their primary caregivers. Transcribed production data showed that 3Psg masculine subject pronoun case was correlated with agreement marking, whereas 3Psg feminine subject pronoun case was not. This result suggests the influence of a retrieval factor, termed the DOUBLE-CELL EFFECT, on the her for she pronoun case error. At the utterance level, pronoun case was independent of the presence of agreement. Overall, the study indicates that the relationship between case and agreement may be discernible as a general correlation, yet indiscernible at the level of sentence production.