Single unit activity was recorded from the hippocampus while Japanese monkeys (Macaca fuscata, n = 4) were performing a delayed response (DR) task. A total of 272 units showed an obvious change in discharge rate in relation to the events of the DR task. These 272 related units were classified into 6 groups: cue-light related units (n = 24), cue- and choice-light related units (n = 41), choice-light related units (n = 21), response-related units (n = 51), reward-error units (n = 17), and delay units (n = 118). Reward-error units contained reward-related and error-related units. Error-related units showed changes in firing after incorrect responses and/or after omission of reward on correct trials. It is noteworthy that 43.4% of the related units are delay units which showed increased or decreased firing preferentially during the delay period. Some units showed a differential firing pattern during cue or delay period depending on the spatial position of the cue. The results of the present study are interpreted as an experimental evidence for the involvement of the hippocampus in DR task.