Krashen's distinction between acquisition and learning has been discussed widely in the recent ESL literature. Reported here are two studies of oral performance by university ESL students which show patterns of use of the present tense third person singular morpheme(‐s)that are interpreted in terms of this distinction. In the first study, speech taken to represent the acquired language system was elicited by asking subjects to talk about several pictures. Analysis of correctly and incorrectly formed third person singular verbs showed that subjects tended to cluster thes's on a few verbs rather than randomly distributing them over all verbs. This pattern suggests that the morpheme may be acquired, not in isolation, but attached to particular verbs. The second study investigated this pattern further by looking at seven third person singular verbs in imitated sentences. Here, just under half of the subjects produced the non‐random cluster pattern. The remaining subjects attached the s more randomly to all verbs in a pattern that most likely represents application of a learned rule (monitoring). A difficulty index constructed for the verbs used in the non‐random pattern suggests that phonetic environment and perhaps frequency of use influence the order in which verbs attracts. Implications for the classroom are disc