The present study examined the pattern of deletion of final /t/ and /d/ in word final consonant clusters in sixteen three- and four-year-old children and their degree of mastery of phonological and grammatical constraints to answer the following questions: how and when is this variable rule acquired, and how does its acquisition relate to the learning of the categorical rule of past tense formation? Sixteen children were tape recorded in their South Philadelphia day care centre. In addition, eight of their mothers were interviewed in their homes for purposes of comparison. Results of the study revealed that children as young as three had, for the most part, mastered the phonological constraints on (-t, d) deletion. They matched the adult pattern including the constraint of following pause disfavouring deletion, the only one that has been shown to vary according to geographical dialect. The children also made a consistent and adult-like distinction between the grammatical forms of mono-morpheme and weak past tense. Their high rate of deletion in semi-weak verbs, which differs from adult patterns, suggests that the children are demonstrating rule acquisition based on an analysis of verbal inflection.