The matrinxã (Brycon cephalus) is an abundant and economically important fish found in the Amazon river basin in Brazil. The seasonal morphological changes in the testes of this fish were studied. Captive-bred specimens were obtained from a breeding center between March 1994 and February 1996. The testes were classified as being of the tubular, unrestricted spermatogonial type, in which four phases of germ cells (spermatogonia, spermatocytes (primary and secondary), spermatids and spermatozoa) were identified. Based on the macro - and microscopic analyses of testes, and on the gonadosomatic index, four stages of germ cell development (resting, maturation, mature and regression) were identified.