Vocabulary diversity is a measure either of the language spoken within a fixed time period or of the total utterances, sentences, or words. To measure children's language development as revealed by the diversity of their spoken expression, a comparison was made of the vocabulary of 15 first-grade children of average ability, of first-grade primers from one of 15 basal reader series, and of 15 first-grade trade books. Vocabulary diversity was determined by both the type-token ratio--in which the number of different words found in a sample is divided by the total number of words in the sample--and by the number of words used only once within each sample. All samples were approximately 500 words. Oral, primer, and tradebook language samples were computer analyzed for calculating the type-token ratios. For each sample among the three vocabularies, 15 type-token ratios were calculated and used for analysis of variance. Findings showed greater diversity in children's vocabularies than in the primers' samples, with no significant difference between oral and trade book vocabularies. References, a sample questionnaire, and tables are included. (JM)