A group of three-year-old children was compared to one of four-year-old children in the usage of 26 syntactic transformations on the basis of 60 utterances per child. The older group used significantly more sentence transformations per child and significantly fewer simple active declarative sentences than the younger. Among the older group 10 out of the 26 transformations were more frequently used, suggesting that they were expanding sentences by use of the auxiliary verb be and producing many more double base transformations. The results suggest that children seem to mature in linguistic competence by acquiring syntactic rules in this order: phrase structure, simple transformations, generalized transformations.