C. nigrofasciatum typically breed in monogamous pairs with biparental care of the young for up to 6 weeks. In experimental ponds in southern Canada and streams in north-western Costa Rica two variations on this system were studied: male desertion followed by maternal care, and simultaneous bigamy by males. In the ponds, with a female-biased sex ratio, males deserted or became bigamous when their broods were at all stages from eggs to advanced, free-swimming fry. These departures from monogamy did not consistently influence brood survival, except that broods deserted by their father shortly after spawning did not survive, whereas those deserted when they were at least one week into the fry stage survived to independence with only their mother as guardian. Both in the ponds and at the field sites females with monogamous partners spent more time away from their broods than did deserted females, but only in the ponds did this correspond with reduced feeding by the deserted females. In nature all parental females foraged less often than did non-parental females, and this reduction was not strongly influenced by mate desertion. Brood defense behaviour by adults in monogamous pairs provided some evidence of division of parental labour. In the ponds guarding females attacked sub-adult juveniles more than males did, whereas in the field guarding females attacked non-cichlid fishes (mainly characins and poeciliids) and conspecific females and juveniles more than males did. Both in the ponds and the field guarding males attacked conspecific adult males more than females did. Most deserted, brood-guarding females seen in Costa Rican streams had young that were relatively large, mobile fry, approaching independence. This may reflect the need for both parents to guard their young during the first few days of free-swimming. They are probably highly vulnerable to predators at this stage and paternal desertion may be more costly than later, when they are stronger swimmers and better able to escape from predators.