The buccal morphology was compared between the sexes of the cardinalfish Apogon doederleini, in which males provide mouthbrooding. The brood size increased proportionally with male buccal space, which increased with the fourth power of the standard length. In the breeding season, males had a larger buccal space than females, whereas there was no sexual difference in the non-breeding season, suggesting sexually different flexibility in the buccal morphology. In spite of a selective advantage to males with a larger mouth, they did not show a higher allometric growth of buccal characters or higher body growth than females. In males, the urohyal was shorter and its height to length ratio was greater than in females. This osteological modification, accompanied by depression of the lower jaw and abduction of the suspensorium, would allow males to expand their buccal cavity more effectively.