Specimens belonging to 20 species (7 families) of catfishes have been collected in French Guyana. The anatomy and histology of the male reproductive tracts have been studied. In the most primitive species (Helogeneidae, Ariidae) no seminal vesicle is present in the males. In the Pimelodidae, Loricariidae and Callichthyidae (and also in an african Bagridae), there is a progressive transition, going from anterior to posterior in the male tract, from testicular tissue towards round germ cell-free tubules and finally to seminal vesicular tissue. All these families produce round-headed spermatozoa and have external fertilization. The Auchenipteridae and Ageneiosidae are characterized by elongated sperm nuclei, internal fertilization with the aid of a gonopodium and sperm storage in the ovaries. In the Auchenipteridae there is a clear-cut distinction between testes and seminal vesicles. In the Ageneiosidae there is no seminal vesicle, but the dilated deferent duct displays a high secretory activity. Depending on the family and species, the seminal vesicles may carry out either one or both of the two main functions: secretion and sperm storage. In addition, in some species, sperm is also stored in one or two accessory pouches until fertilization. The trends that we have observed in the changes in the organization of the reproductive tracts are in agreement with the hypotheses proposed for catfish evolution from comparative anatomical data.