We investigated the endocrine basis for sexual dimorphism of the sound-producing apparatus (swimbladder and attached sonic muscles) in the oyster toadfish Opsanus tau by implanting steroid pellets in gonadectomized females and males. In females, testosterone and dihydrotestosterone caused, respectively, 32.3 and 31.5% increase (P less than 0.0001), and estradiol 17 beta caused 11.3% increase (P less than 0.0104) in swimbladder weight compared with controls. In males, testosterone caused 17.7% (P less than 0.028), dihydrotestosterone 24.0% (P less than 0.0008), and estradiol 10.8% (N.S.) increase from controls. Swimbladder weight gains occurred despite the fact that these fish refused food and did not gain weight. In a final experiment on fed females, testosterone and 11-keto-testosterone caused a similar increase (16.6 and 18.9%, respectively) compared with controls. These results indicate that swimbladders in adults of both sexes retain the ability to respond to steroid treatments and suggest that sexual differences in bladder size are controlled without a critical period by hormonal concentrations within the fish.