A field study of the pigtail monkey (Macaca nemestrina) was conducted in Malaysia. An initial six month survey revealed a badly disturbed, diminishing population. Intensive observation required prolonged extensive habituation efforts including a modified form of provisionization and the use of a trained captive pigtail monkey. The pigtail monkey was found to be an arboreal deep forest animal which, nonetheless, came to the ground more often than any other monkey in the forests of Malaya. Responses observed in the natural habitat could not be differentiated from those seen in captive groups; but response frequencies differed. Social patterns observed were similar to those seen in other macaques but differed notably from those seen in sympatric leaf monkeys. Travel patterns, feeding, agonistic responses, sexual behavior, play and other responses are described and discussed.