The original Bayley Scales of Infant Development (BSID) have been among the most popular measures of performance and aptitude of infants. In this study, the construct validity of scores on the Behavior Rating Scale of the revised Bayley Scales, the BSID-II, was investigated using national standardization and clinical samples of children ranging in age from roughly 1 to 42 months, and a variety of factor analytic methods. In all, 2,106 children (562 1- to 5-month-old, 503 6- to 12-month-old, and 1,041 13- to 42-month-old subjects) provided data for the analyses. Results indicate that motor performance has an important influence on scores on the BSID-II Behavior Rating Scale, a result that is judged to be consistent with expectations. It is also clear that the structure underlying scale scores becomes increasingly more complex as the samples become more heterogeneous. The least heterogeneous samples are at the youngest age where development is least differentiated. These results offer insight into the integrity of scores from the new Bayley Scales. Seventeen tables present analysis details. (Contains 26 references.) (SLD)