FONTOURA, Nelson Ferreira; AGOSTINHO, Angelo Antonio. Growth with seasonally varying temperatures: an expansion of the von Bertalanffy growth model. Journal of Fish Biology, London, v.48, no.4, p.569-584, 1996.The von Bertalanffy growth function has limitations for describing the growth of fishes in seasonal climates. In the present work, a new equation is proposed where the growth parameter k is substituted by a function related to monthly water temperatures. The computer program GROWTH5 was developed to fit and simulate the growth for seasonally varying temperatures. Examples for natural populations of Barbatula barbatula and Cottus gobio are presented.