The locomotory activity and reproductive behaviour of the Philander opossum Caluromys philander have been studied in a 80-year old forest near Cayenne, French Guyana, from September 1978 to October 1979. One hundred and seventy one individuals were live-trapped and marked, and 12 of them (5 ♀♀ and 7 ♂♂) followed by radio-tracking. This solitary, arboreal and nocturnal marsupial is predominantly frugivorous, a few arthropods supplementing a basically vegetarian diet. Its home range averages 0.75 ha (N = 26) and broadly overlaps those of neighbouring individuals. Some individuals are very sedentary ; one of them has been recaptured 43 times in the course of the year in the study area. Others are more mobile ; two of them equipped with radio-transmitters have wandered 500 metres away from the area where they were first captured. Some immature and adult transients have also been noted in the study area. Significant variations in the duration of the nocturnal activity periods have been observed ; they were correlated with the availability of food supply and the reproductive status of the females. Lactating mothers with pouch-young older than 5 weeks (Group 1) needed twice as much food as non-lactating females, or females with pouch-young less than 5 weeks old (Group 2). From November to May, when food supply was abundant, Group 1 females were active on average during 9 hours every night, as opposed to 5 hours per night in Group 2 females. From June to October, when food resources were scarcer, females of the two groups spent more than 9 hours per night foraging for food. Two breeding periods were noted during the year of study. The first took place from October to December 1978. The food situation being very favourable at that time, 95 % of the young born survived until weaning. A second generation was produced between April and June 1979, at a time when food was becoming scarcer. The reproductive success was consequently much reduced, over 80 % of the young dying in the pouch, and mothers losing weight. Such “ pouch-abortions ” were followed by a two-month anoestrous period. A third batch of young were born in September 1979, just before the end of the field study. As other Didelphids, Caluromys philander is a polyoestrous marsupial. However, scarcity of food can lead to “ pouch abortions ” and to a temporary inhibition of reproduction. bility of food supply and the reproductive status of the females. Lactating mothers with pouch-young older than 5 weeks (Group 1) needed twice as much food as non-lactating females, or females with pouch-young less than 5 weeks old (Group 2). From November to May, when food supply was abundant, Group 1 females were active on average during 9 hours every night, as opposed to 5 hours per night in Group 2 females. From June to October, when food resources were scarcer, females of the two groups spent more than 9 hours per night foraging for food. Two breeding periods were noted during the year of study. The first took place from October to December 1978. The food situation being very favourable at that time, 95 % of the young born survived until weaning. A second generation was produced between April and June 1979, at a time when food was becoming scarcer. The reproductive success was consequently much reduced, over 80 % of the young dying in the pouch, and mothers losing weight. Such “ pouch-abortions ” were followed by a two-month anoestrous period. A third batch of young were born in September 1979, just before the end of the field study. As other Didelphids, Caluromys philander is a polyoestrous marsupial. However, scarcity of food can lead to “pouch abortions” and to a temporary inhibition of reproduction.