The adequacy of mean length of utterance (MLU) as a criterion for matching retarded and normally developing children was investigated. Six normally developing and four retarded subjects were matched on MLU. Although matched on MLU, these two groups of children demonstrated both quantitative and qualitative differences in their use of language. Normally developing children produced a higher rate of intelligible speech and a greater proportion of that speech was devoted to spontaneous and multi-word utterances. In addition, these normally developing children demonstrated larger spontaneous vocabularies than the mentally retarded children. It appears that these differences were not simply a function of rate of intelligible speech. It is concluded that the data of this study support the argument made by Klee (1985) that MLU does not discriminate children's profiles of grammatical development. Linguistic matching of children demands multiple measures of child language. (Author/RH)