Cichlids of the genus Oreochromis have a duplication of the prolactin gene, and the two paralogs have diverged substantially in sequence. Cichlids as a group show elaborate patterns of obligate parental care, and Oreochromis is characterized by costly female parental behavior. Recent experiments indicate a role for prolactin in stimulating parental care behaviors in some species of fish, particularly sticklebacks. We hypothesize that the recent paralog of the prolactin gene in Oreochromis (and possibly other cichlids) plays a role in the elaborate parental care of these fish and has diverged due to selection on this function since the duplication event. We provide a preliminary test of this hypothesis by searching for a signal of positive selection on the branch leading to the recent paralog clade, and by comparing the average ratio of nonsynonymous to synonymous substitution rates in the two paralog clades. Our analysis revealed significant evidence for positive selection on the new paralog after gene duplication.