This research study evaluated the effects of memory demand on sustained attention, by varying the time interval between the presentations of stimuli in a cognitive vigilance task. Sixteen retarded and 16 nonretarded adult observers performed four 60-minute visual vigilance tasks in which single digits were presented sequentially and successively at either a high rate (30 events per minute) or a low rate (15 events per minute). During two of the four tests, the event rate presented during the initial 30 minutes was shifted without warning to the alternate event rate for the final 30 minutes. Results of the study indicated the presence of a vigilance decrement in the performance of retarded observers during both event rate conditions. Retarded adults detected fewer targets and made many more false alarm responses than did nonretarded adults. It is concluded that educational programs need to be aware that adults with mental retardation attend to crucial information less often than the nonretarded, and they tend to make more false positive responses. (JDD)