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Given climate change projections, the limited ability of fish reared in sea-cages to behaviourally thermoregulate, and that thermal tolerance may be heritable, studies that examine family-related differences in upper thermal tolerance are quite relevant to the aquaculture industry. Thus, we investigated the upper thermal tolerance of 15 Atlantic cod (Gadus morhua L.) families by challenging them with acute (2 degrees Ch(-1)) and incremental (1 degrees C every 4 days) temperature increases (CTmax and ITmax tests, respectively) under normoxia (similar to 100% air saturation) and mild hypoxia (similar to 75% air sat.). The cod's CTmax was 22.5 +/- 0.1 degrees C (mean +/- S.E.) during normoxia and 21.8 +/- 0.1 degrees C during hypoxia (P < 0.001); and these two CTmax values were significantly correlated across families. In both the normoxic and hypoxic ITmax tests, feed intake fell by similar to 50% between 17 and 18 degrees C, and stopped entirely by 21 degrees C mortalities were observed under 20 degrees C in the normoxic and hypoxic ITmax tests, and the ITmax value was similar to 21.7 degrees C in both groups. Differences in the upper thermal tolerance between families were only observed in the CTmax experiment. No correlation was found between the specific growth rate and the CTmax of the families. Further, no correlation existed between CTmax and ITmax. This study is the first to compare the thermal tolerance of fish families to both CTmax and ITmax challenges, and the data: 1) suggest that the Atlantic cod is quite tolerant of acute (i.e., hours) or short-term (i.e., weeks) exposure to high water temperatures (i.e., up to 20 degrees C); 2) indicate that it might be difficult to select fish with higher ITmax values; and 3) question the relevance of CTmax for selecting fish that are destined for sea-cages where temperatures slowly warm over the summer.