Male bluegill sunfish (Lepomis macrochirus) exhibit alternative life histories: some males (parentals) delay maturation for up to 7 yr, then build nests, court females, and care for the eggs and fry, whereas other males (cuckolders) mature precociously, attempt to steal fertilizations from parentals, and provide no parental care. Parental males could avoid misdirecting their nepotism (i.e. caring for unrelated young) by abandoning entire broods if they were sired mainly by cuckolders or by discriminating between offspring and non-kin fry within broods for which they care. We tested for kin discrimination by obtaining sperm from parental and cuckolder males and eggs from several females, and using them to conceive fry in vitro. In ‘blind’ laboratory tests, parental males (but not cuckolder males) distinguished between sources of dripping water that had been conditioned by their own offspring vs. unrelated fry. Parental males that were in the best physical condition were especially choosy. Because the only referents available to our experimental subjects were chemical cues emanating from their own body, our results imply that parental males can use self-referent phenotype matching for kin recognition. This mechanism enables males to make the adaptive, nepotistic adjustments in paternal care that have been documented in previous studies.