Critical thermal minima (CTMin) and maxima (CTMax) values were determined for the Pacific white shrimp Litopenaeus vannamei post-larvae and juveniles at four different acclimation temperatures (15, 20, 25, and 30 C). The CTMin of shrimp at these acclimation temperatures were 7.82, 8.95, 9.80, and 10.96 degrees C for post-larvae and 7.50, 8.20, 10.20, and 10.80 degrees C for juveniles, respectively, at 1 degrees C h(-1) cooling rate. The CTMax values were 35.65, 38.13, 39.91, and 42.00 degrees C for post-larvae and 35.94, 38.65, 40.30, and 42.20 degrees C for juveniles at the respective acclimation temperatures. Both acclimation temperature and size of the shrimp affected CTMin values of L. vannamei (P < 0.01). Overall, juveniles displayed significantly lower CTMin values than the post-larvae (P < 0.0001). However, the CTMax response by post-larvae and juveniles were not significantly different from each other and no interaction was determined between the acclimation temperature and development stage (P > 0.01). The area of the thermal tolerance polygon over four acclimation temperatures (15, 20, 25, and 30 degrees C) for the post-larvae of L. vannamei was calculated to be 434.94 degrees C-2. The acclimation response ratio (ARR) values were high ranging from 0.35 to 0.44 for both post-larvae and juveniles. L. vannamei appears to be more sensitive to low temperatures than other penaeid species and its cold tolerance zone ranged from 7.5 to 11 degrees C. In successful aquaculture temperature must never fall below 12 degrees C to prevent mortalities. Upper thermal tolerance is less of a problem as in most subtropical regions maximum water temperature rarely exceeds 34 degrees C, but care should be given if shallow ponds with low water renewal rate are being used. (C) 2010 Elsevier Ltd. All rights reserved.