A cannibalism in the larvae of a salamander (Hynobius retardatus) and a resultant dimorphism were firstly described in laboratory-raised population. The cannibalistic larva develops to a “broad-headed” morph which has a wider head and grows faster compared with a non-cannibalistic “typical” morph. Laboratory experiments were conducted to examine a possible relationship between the dimorphism and cannibalism. Level of food supply and kinship of larvae affected the frequency of cannibalism. Low level of the food supply resulted in higher incidence of the cannibalism. The frequency of cannibalism in siblings was significantly lower than in the mixed-sibling groups. Possible biological significance of the cannibalism in this species was discussed.