The reproductive biology ofPatagonotothen tessellata(Richardson 1845) in the Beagle Channel, Tierra del Fuego, Argentina was analysed from weekly captures over a 3‐year period (1987–1989). The spawning periods and lengths thereof were analysed using GSI curves and monthly percentage distributions of maturation stages. Two gonadal maturation cycles and two spawning periods (winter and summer) within 1 year were defined. Also discussed is whether the same specimens are able to mate successfully in winter and summer, and whether the reproductive effort varies between the seasons. Male parental behaviour and the likely consequences on their energy levels are conside