Part of a longitudinal study of women and their first-born children who have different employment and day care experiences during the children's first three years of life, this study focuses on the socioemotional development of children at 18 months of age, who had received at least nine months of continuous day care. Particular attention is given to the effects of group care in day nurseries, individual care by a relative, individual care by a childminder, and home care by the mother. Data for 166 children were obtained from three measures: a stranger approach/separation/reunion sequence; the first seven items of the Infant Behavior Record, which uses ratings to describe children's behavior during the administration of the Bayley Mental Development Index Scale; and a socioemotional development questionnaire, which was designed to obtain data on children's behavior from the mother's experience. While not supporting parents' belief that nursery care would improve their children's sociability, findings indicate that day care experience did significantly affect children's socioemotional development. Children in nursery care showed less sign of pleasure when approached by a stranger than children in the other groups. Upon separation from the mother in the presence of the stranger, children's concern increased across home, relative, childminder, and nursery groups. Infant Behavior Record data from nursery group children showed that they were less oriented than others to people and were more negative in mood. The socioemotional development questionnaire showed no effects associated with day care experience. (RH)