Ten young males whose IQ scores ranged from 51 to 71 were compared on a simple discrimination task with 10 male university students of the same CA and 28 nonrelated male children. Estimates of inspection time (lambda) were obtained from the pattern of errors made by each subject; lambda is assumed to reflect the rate at which sensory input is accumulated and processed. Average estimates of lambda from four groups of children (7, 8, 9, and 10 years) were 147, 142, 137, and 139 msec, respectively. The mean for university students was 130 msec, whereas that for retarded adults was 256 msec. Mean reaction time (RT) of all nonretarded groups increased as stimulus-exposure duration decreased, children being slower than adults. Retarded subjects' RT was less influenced by stimulus-exposure duration and was faster than that of nonretarded groups at the shortest exposure. These results suggest that slower perceptual speed among retarded persons is a consequence of a permanent deficiency; differences in response strategy may explain the differences in RT.