Behavioral similarity between partners is likely to promote within-pair compatibility and to result in betterreproductive success. Therefore, individuals are expected to choose a partner that is alike in behavioral type.However, mate searching is very costly and does not guarantee finding a matching partner. If mismatchedindividuals pair, they may benefit from increasing their similarity after pairing. We show in a monogamous fishspecies—the convict cichlid—that the behavioral similarity between mismatched partners can increase afterpairing. This increase resulted from asymmetrical adjustment because only the reactive individual becamemore alike its proactive partner, whereas the latter did not change its behavior. The mismatched pairs thatincreased their similarity not only improved their reproductive success but also raised it up to the level ofmatched pairs. While most studies assume that assortative mating results from mate choice, our study suggeststhat postpairing adjustment could be an alternative explanation for the high behavioral similarity betweenpartners observed in the field. It also explains why interindividual behavioral differences can be maintainedwithin a given population.