This paper describes a study which examined the performance of 48 pairs of 18-month-old twins on the Mental Development Scale of the Bayley Scales of Infant Development to determine whether score differences would be found for the three subgroups of identical, fraternal same-sexed, and fraternal opposite-sexed twins. Of the 96 subjects, 46 (23 pairs) were identical twins; 22 subjects (11 pairs) were same-sexed fraternal; and 28 (14 pairs) opposite-sexed fraternal twins. The twins were tested in their homes, generally, within two weeks of their 18-month birthday. For the entire group, the overall Mental Development Index was 95.5; for identical twins, the average Index was 94.0; for same-sexed fraternal twins, the average Index was 90.4; and for opposite-sexed twins, the average Index was 102.2. These results are discussed, and it is suggested that the typical differential treatment given to boys and girls may lead to a greater differentiation of treatment and/or more individual attention for the opposite-sexed twins than for same-sexed twins. In addition, twin-pair interaction may differ for same-sexed and opposite-sexed twins, influencing each twin's intellectual development in different ways. (CS)