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The ability of rainbow trout (Oncorhynchus mykiss) spermatozoa to withstand cryopreservation was tested after the fish and sperm were exposed to various temperature conditions. In the first experiment, trout were acclimated at either 8 °C or 18 °C during gametogenesis, and transferred to 13 °C at the beginning of the spawning season. Following this 13 °C transfer, the fertilizing ability of frozen-thawed sperm from the fish reared at 18 °C was found to be twice as better (60–80% fertilization rate) than that of the fish reared at 8 °C (20–30% fertilization rate) up to 42 days after the transfer. A low membrane cholesterol/phospholipid ratio was strongly correlated (P < 0.002) with the better freezing resistance of the corresponding sperm. The relevance of this parameter in predicting sperm ability to survive cryopreservation is discussed. In another experiment, fish with the same thermal background during gametogenesis …