This study compared the language skills in a group of very low-income toddlers with those of a middle-income sample matched on age and sex. The assessment instrument was the MacArthur Communicative Development Inventory (CDI) for toddlers, a parent report form. The scores for the low-income group were strikingly lower on the three key indices evaluated: size of expressive vocabulary, age of appearance of word combinations, and complexity of utterances. The entire lowincome distribution was shifted about 30% toward the lower end of the middle-income distribution for both productive vocabulary and grammatical development. The magnitude of these income/ social class effects was larger than reported in most prior reports for children in this age range. This finding underscores the cautionary note issued by the CDI developers, which states that the published CDI norms, based on a middle-class sample, may not be directly applicable to low-income samples.