Male Pygmy Sculpins (Cottus pygmaeus) were found to abandon nests that contained clutches of eggs, presumably establishing new nests elsewhere. These abandoned nests were readily occupied by new males (allopaternal males) that continued paternal care of existing clutches and usually acquired new clutches. Significantly more eggs were lost to cannibalism when new males acquired nests than when the original males stayed with their own brood. The size of males that abandoned nests did not differ from that of allopaternal males. Female preference for males with nests that already contain eggs may be the selective force for the occurrence of allopaternal care in this species.