Variation in mothers' child-directed speech and in their children's rates of language development were examined as a function of child birth order and family socioeconomic status (SES). A total of 63 children between 18 and 29 months were recorded in dyadic interaction with their mothers on two separate occasions, 10 weeks apart. The children included first and later boms who came from high-SES and mid-SES backgrounds. Analyses of the children's speech at the second visit showed that the first-bom children were more advanced in lexical and grammatical development than the later-bom children, and that the later-bom children were more advanced in the development of conversational skill. High-SES children showed more advanced lexical development than mid-SES children. These differences are interpreted as the result of differences in language learning experience associated with birth order and SES, some of which were in evidence in the mothers' speech recorded at the first visit. With respect to theories of language acquisition, these findings suggest that language experience plays a nontrivial role in language development, and that the nature of that role is different for different components of language development. With respect to general developmental consequences of birth order and SES, the findings indicate that differences in early language experience may set the stage for later developmental differences, but that when long-term and pervasive differences are observed, as is the case for SES-related differences in achievement, it is likely that there are pervasive and continuing differences in experience.