Using homologous molecular probes, we examined the influence of equivalent temperature shifts on the in vivo expression of genes coding for a constitutive heat shock protein (Hsc70), heat shock proteins (Hsps) (Hsp70 and Hsp90), and polyubiquitin, after acclimation in the American lobster, Homarus americanus. We acclimated sibling, intermolt, juvenile male lobsters to thermal regimes experienced during overwintering conditions (0.4 ± 0.3°C), and to ambient Pacific Ocean temperatures (13.6 ± 1.2°C), for 4–5 weeks. Both groups were subjected to an acute thermal stress of 13.0°C, a temperature shift previously found to elicit a robust heat shock response in ambient-acclimated lobsters. Animals were examined after several durations of acute heat shock (0.25–2 hours) and after several recovery periods (2–48 hours) at the previous acclimation temperature, following a 2-hour heat shock. Significant inductions in Hsp70, Hsp90, and polyubiquitin messenger RNA (mRNA) levels were found for the ambient-acclimated group. Alternatively, for the cold-acclimated group, an acute thermal stress over an equivalent interval resulted in no induction in mRNA levels for any of the genes examined. For the ambient-acclimated group, measurements of polyubiquitin mRNA levels showed that hepatopancreas, a digestive tissue, incurred greater irreversible protein damage relative to the abdominal muscle, a tissue possessing superior stability over the thermal intervals tested.