Previous research on the Antarctic notothenioid fish, Trematomus bernacchii, demonstrated the loss of the heat-shock response (HSR), a classical cellular defense mechanism against thermal stress, characterized by the rapid synthesis of heat-shock proteins (Hsps). In the current study, we examined potential mechanisms for the apparent loss of the HSR in Antarctic notothenioids, and, in addition, compared expression patterns of two genes from the 70-kDa Hsp family (hsc71 and hsp70) in tissues from T. bernacchii to expression patterns in tissues of two closely-related temperate notothenioid fishes from New Zealand- Bovichtus variegatus and Notothenia angustata. The results showed that transcript for both the constitutive and inducible genes in the Hsp70 gene family were expressed in detectible levels in all three species. However, only the cold-temperate New Zealand fishes displayed the ability to up-regulate the inducible transcript, hsp70. Although hsp70 was present in detectable levels in several tissues of the Antarctic notothen T. bernacchii, in vitro thermal stresses failed to produce a significant increase in mRNA levels. In all species, the expression of the constitutive transcript, hsc71, was variable and non-responsive to temperature increases, even at temperatures as high as 10 °C above the ecologically relevant range for the species under study. Field-collected tissues from T. bernacchii (sampled immediately after capture) indicated that hsp70 mRNA was expressed at high levels in field-acclimatized fishes. Thus, up-regulation of molecular chaperones suggested that low-temperature stress may be significantly denaturing to cellular proteins in Antarctic fish, an observation that was supported by elevated levels of ubiquitin-conjugated protein.