Children's acquisition of agent nouns within a framework of morphological structural principles is explored. Language acquisition has been conceptualized as a process of parameter setting in which the learner is richly endowed with a vocabulary of primitives and rule schemata. Exposure to the primary data will be filled in from the range of options allowed by universal grammar. It is argued that while it may initially appear that learners are using rules that are incorrect for analyzing the particular data, these rules actually fit into the larger system the learners are acquiring. Children's use of morphological structural principles in agent noun acquisition was evaluated in an experiment involving 21 children between the ages of 3 and 7. The results were consistent with the hypothesis that children's grammars reflect morphological structural principles within a framework which takes into account the logic of learnability. (RW)