The reliability and validity of the behavior of a sample of institutionalized mentally retarded children, with total IQs between 2 and 53, were assessed by a new scale measuring the following areas; communication, self-help, physical skills, self-control, and social behavior. Clinically and statistically acceptable levels of interrater agreement were found. With respect to validity: (a) several factors loaded very highly on items consistent with the a priori labeling of subscales; (b) two of the factors (cognitive and psychomotor development) could account for about 75% of the explained variance in ward placement levels (multiple R = .86); and (c) individual items on the Behavior Rating Inventory for the Retarded correlated highly with independently derived measures of the actual behavior these items represented. The major advantages of this scale over existing ones include its clearly demonstrated validity and relative ease of administration and scoring.