The effect of grammatical phonological rules (those in English) and nongrammatical (those in other languages) on the learning and reproduction of morpheme-length utterances and the role of maturation on this effect were examined. Children preschool through second grade were the subjects. There were no significant differences at any grade level between the percentages of children learning sets of grammatical or nongrammatical sequences. There were significant differences at each grade level in reproducing the two categories of sets. The task of reproducing nongrammatical sequences was much more difficult, and the time needed for processing nongrammatical sequences in both tasks was consistently longer. It was concluded that nongrammaticality of phonological sequences markedly affects reproduction at an early age, but not perception and storage. (Author/FWB)