Two fungal parasites isolated from the fish Odontesthes bonariensis in Argentina were identified as Achlya racemosa and Saprolegnia ferax based on morphology and ITS rDNA sequence comparison. Studies were conducted to determine the effect of culture temperature and culture medium on growth rate and stimulation of sexual reproduction. Both isolates showed an increased rate of vegetative growth with increasing temperature; the growth rate of Saprolegnia ferax was higher than that of Achlya racemosa. Culture medium did not significantly affect growth rate of either species at the same temperature. However, differences attributable to culture medium in the formation of sexual structures were detected. © 2011 J. Cramer in Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany.