Several Old World monkeys (M.fascicularis) have demonstrated their ability to remember, after delay intervals of many secs., various sequences of acoustic stimuli. Both the nature of a given sequence as well as the order of the two elements which comprised it were shown routinely by the animals to have been retained at>90% correct response levels during months of daily test sessions [J. H. Dewson III, Acoust. Soc. Am.57, S11 (A) (1975)]. Detailed analysis of the errors made on this complex conditional match‐to‐sample task reveals the effects on memory of strong proactive interference (PI) when the elements of a sequence are of equal (0.5 sec) duration. By selectively altering the relative durations of the two elements, the situation can be reversed to one in which retroactive interference (RI) is clearly shown. This demonstration of PI and of RI in individual animal subjects, obtainable within single testing sessions, provides an indication of basic similarity of memory processes across the primate order.