Rainbow trout were subjected to four thawing treatments after being stored at -18°C and -40°C for 0, 3, 7, 13 and 18 mo. Membrane integrity was estimated as the volume of centrifuged tissue fluid (CTF) and by lysosomal (3-N-acetylglucosaminidase (NAG) activity in CTF. Slow thawing, in air at 5°C, resulted in higher NAG activity in CTF and a larger volume of CTF than fast thawing, at 25°C in water, independent of storage time. After 3 or more months storage, a higher NAG activity in CTF and a larger volume of CTF were found in all - 18°C stored samples compared to that at -40°C. Sensory evaluation confirmed differences between trout stored at - 18°C and at - 40°C for 18 mo.