A gonadotropic hormone of the African catfish, Clarias gariepinus, was was purified and chemically characterized. Its biological activity was tested and its localization in the gonadotropic cells of the pituitary demonstrated. An ethanolic extract of 500 pituitaries of adult male and female African catfish was subjected to ion-exchange chromatography on DE-52. The 31- to 38-kDa fraction was further purified on Sephadex G-75. On rpHPLC over an ODS 120T column two major components appeared as single bands after SDS-PAGE. From the amino acid composition and sequence analysis of these fractions, compared with those of salmon and carp GTH II-alpha and salmon GTH II-beta it was concluded that they represent catfish GTH alpha- and II-beta-subunits. The biological activity of the complete hormone (the 31- to 38-kDa fraction from the G-75 column) was tested on the production of 11 beta-hydroxyandrostenedione and 17 alpha-hydroxy-20 beta-dihydroprogesterone by catfish testis in vitro. Polyclonal antibodies were raised against the purified beta-subunit. Immunocytochemical study using these showed them to bind specifically to hypophysial gonadotropic cells. To date only one form of GTH has been demonstrated in the African catfish.