The acquisition of verb morphology is often studied using categorical criteria for determining the productivity of a morpheme. Applying this approach to Japanese, an agglutinative language, this study finds no consistent order for morpheme acquisition and that productivity could be explained by sampling effects. To examine morpheme acquisition using more graded measures of productivity, the authors compared various regression models for predicting the age of acquisition of 311 verb forms across a large combined corpus of seven Japanese-speaking children (aged 1;1-5;1). Complex forms were learned earlier than frequency-matched simple forms, and morpheme ending identity explained substantial variation. Both of these findings suggest that children have some segmented morphemes and have learned some of their semantic/pragmatic characteristics. Sampling would predict that verb form acquisition would be sensitive to lemma and ending frequency, but acquisition was also sensitive to aspects of input frequency that were independent of these factors, and this suggests that children are encoding whole verb forms in addition to creating forms with compositional morphological rules.