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There is little experimental evidence testing whether current brood size and past brood mortality influence mate desertion. In the cichlid Aequidens coeruleopunctatus both parents initially defend offspring. In a field study, all experimental broods, irrespective of initial brood size (222.9 +/- 60.4, mean +/- SD), were manipulated to a size of 100 fry. Neither the duration nor investment of females in parental care differed between control and brood reduced pairs, even though care seemed costly. On average, females lost 5.1 +/- 4.8% of initial weight while guarding a brood until independence. In contrast, males with experimentally reduced broods guarded fry for significantly fewer days before deserting their mate than did males from control pairs,vith natural-sized broods (20.5 +/- 7.5 vs. 14.2 +/- 6.2 days). In at least 20% of cases (n = 9/45), the deserting male immediately mated with another female. Males with experimentally reduced broods also spent less rime guarding fry before deserting and attacked fewer brood predators than did males with control broods. For broods manipulated to have 100 fry, there was a significant negative relationship between the days until male desertion and the proportion of the initial brood removed. This indicates that male assessment of the future success of the current brood (hence its reproductive value) is based on past mortality and/or that there is variation among males in the expected size of future broods. Both current brood size and brood size relative to initial brood size are therefore predictors of male, but not female, parental behavior and mate desertion. Female care may be unaffected by brood reduction due to limited breeding opportunities and partial compensation for reduced male care.