Data on the reproductive patterns of the Brazilian slender opossum (Marmosops paulensis) were collected in an area of Montane Atlantic forest, southeastern Brazil, from August 2002 to July 2004. Reproduction occurred from September to March in both years, a period of high food supply, probably as a way to maximize survival of juveniles. There was nearly zero postmating survival, thus, no individual took part in more than 1 breeding event. This pattern characterizes a semelparous life history, which has been described in other small didelphids and dasyurids. Females were reproductively active during months with longer day lengths and abundant fruit supply. Breeding seems to be initiated by a 12L:12D photoperiod and a rapid rate of change in day length, as demonstrated in semelparous dasyurids. Hence, the effect of photoperiodic cues on the onset of reproduction also may stand for other semelparous didelphids. We suggest that fruit availability controlled the length of breeding activity in M. paulensis, and it could play a role in the occurrence of semelparity in this species. However, semelparity may occur only due to phylogenetic constraints, whereas food supply works as a selective force maintaining this trait.