Semistructured interviews with 305 mothers of same-sex twins were conducted periodically over the course of the twins' first 6 years. During each of the interviews, the mothers were asked to contrast the twins on a variety of specific behaviors. From the body of reports, 5 ages and 16 behaviors were selected for analysis as most illustrative of the main developmental trends for intrapair differences and similarities. For each of the behaviors, intrapair contrasts were moderately stable across age spans as long as 18-36 months. A set of behaviors related to negative aspects of temperament and a set of behaviors related to sociability were obtained at 6 months and were replicated at later ages. Concordance rates for the identical twin pairs were uniformly higher than for the fraternal pairs, suggesting that genotypic similarity was associated with intrapair similarity for aspects of emotionality and sociability.