Differences in temperament, attention span, and activity level as reported by mothers of 56 pairs of 12-mo.-old twins were related to tested differences in cognitive abilities at age 4 yr. The more active twins were found to show temper more, have shorter attention span in play, and walk earlier but to have poorer manual dexterity than their co-twins. The twins with longer attention showed less temper and walked later but had better manual dexterity than their co-twins. Twins who were less active or had longer attention span at 12 mo. had significantly higher WPPSI Performance IQs than their co-twins when tested at age 4 years. Verbal IQ differences were in the same direction but not significant.