SUMMARY The treatment of female or female phase specimens of Cobitis taenia bilineata Canestrini with 100 μg of methyltestosterone/litre of aquarium water induced the formation of Canestrini's organ and a thickening of the 2nd ray of the pectoral fin. These experimental results, correlated with observations in nature, indicate that Canestrini's organ is a hormonally controlled secondary sex character which develops fully in males and male phase individuals of this partially protandrous hermaphrodite species.