Testosterone in the haemolymph, hepatopancreas and testis were quantified using radioimmunoassay, during different stages of testicular maturation in the marine crab, Charybdis natator. Testosterone was not detectable in the haemolymph, when the testis was in stage I spermatogonia, and it gradually increased during stage II spermatocytes and III spermatids and reached a peak level during stage IV spermatozoa: In the testis, testosterone showed a peak levels during stages III spermatids and IV spermatozoa, respectively.