Cannibalism is common in high density larval populations of Hynobius retardatus. Because cannibals are gape-limited, possessing a wider head (mouth) may be advantageous in these populations. Field observations showed that the pre-feeding stage larvae were more vulnerable to cannibalism than feeding stage larvae in a high-density larval population. The data also showed that larvae with proportionally smaller head widths are more vulnerable to cannibalism than those with larger heads. Therefore, faster growth of head size during pre-feeding stage is predicted to be favored in a population with frequent cannibalism. A laboratory comparison revealed that head growth (proportionate change in head width to body length) during the pre-feeding stage was greater in the larvae of a cannibalistic population than in those of a non-cannibalistic population. These results support the hypothesis that a wide head is an adaptation against frequent cannibalism in larval H. retardatus.