Biparental care is expected to occur if (1) the costs of desertion for a parent are high, because of greatly reduced survival prospects of offspring, or (2) the benefits of desertion are low. Among mouthbrooding fish, biparental care is rare because the mouth cavity provides a safe brooding site, thus reducing the selective advantages of shared brood care. Eretmodus cyanostictus is a monogamous mouthbrooding cichlid in which the entire clutch is brooded first by the female and then by the male. To test the hypothesis that females alone can produce viable young, we designed an experiment in which females were separated from their mates. Unassisted females prolonged incubation but released as many young as females assisted by males. However, they compensated only partially for male incubation and released smaller and less-developed young. This may substantially reduce offspring survival chances in the wild. The body condition of single females decreased more during incubation and they had a prolonged interspawning interval, but produced similar egg numbers and weights in the next clutch. Our results suggest that the male's brood care effort is an important cause of the maintenance of biparental care and monogamy in E. cyanostictus.