Given the many detailed studies on the breeding biology of gulls, surprisingly little is known about conspecific brood parasitism (CBP) in these birds. CBP has been recorded in only a few of the roughly 60 species of Laridae. We studied CBP in two colonies of Common Black-headed Gull Croicocephalus ridibundus in northeastern Poland, during two consecutive years. We found parasitism in 22% and 10% of the monitored nests in the colonies, as determined with protein fingerprinting. Among the parasitic females we found only one that acted as both parasite and host. In both colonies, most cases of parasitism involved nests of females more related than average to the parasitic female. The rare cases of egg expulsion involved hosts with low relatedness to the parasite. We suggest that kin selection is implicated in CBP in the Blackheaded Gull.