It is generally assumed that there is sexual conflict over the mating system. In this view, polygyny benefits males at a cost to females, and it is hence unclear why females accept polygynous mating. However, in the facultatively polygynous fish, Neolamprologus pulcher, no costs of polygyny to females have thus far been detected. We hypothesized that the costs of polygyny remained undetected because they accrue over longer periods of time through reduced tenure and/or survival. We conducted an extended field study in which we monitored the behavior and survival of individuals breeding either monogamously or under polygynous conditions within the same natural colony. We expected that polygyny would reduce male and female survival through increased competition among males and reduced amounts of received paternal effort for females. Consequently, breeder tenure and pair stability were predicted to be lower in more polygynous groups. Our data indeed revealed costs of polygyny to both sexes. Polygynous males faced higher competition, and females paired to polygynous males received reduced paternal effort. However, this did not result in different survival rates between individuals breeding under monogamous or polygynous conditions. We conclude that in N. pulcher the fitness costs of polygyny may be either too marginal to be detected with the approaches used thus far, or that males and females in this species do not face a conflict of interest over the mating system. This raises the question which ecological factors may resolve sexual conflict, and how the accruing mating system feeds back on a species’ ecology.