SUMMARY: The thermal stress responses of a sterile mutant of the marine alga Ulva pertusa were investigated at 20°C and 30°C. The amounts of the photosynthetic pigments, β‐carotene, chlorophylls a and b, lutein, neoxanthin, and violaxanthin, were 1.4–2.4 times higher in the 30°C‐cultivated alga than in the 20°C‐cultivated alga. The free amino acids, asparagine, aspartic acid, glutamine, glutamic acid, glycine, and serine, were abundant in the 20°C‐cultivated alga, and increased 1.9–10.5‐fold in response to thermal stress (30°C). Total carbon and nitrogen contents also increased in the 30°C‐cultivated alga. Sodium dodecylsulfate‐polyacrylamide gel electrophoretic patterns of total proteins extracted from both temperature‐treated algae showed the increases of 20, 25, and 90 kDa proteins in the 30°C‐cultivated alga. Isozyme assays for 20 enzymes showed a different banding pattern only in the case of glutamate dehydrogenase (GDH). Although it was observed that both temperature‐treated algae possessed NAD+‐ and NADP+‐specific GDH, the 30°C‐cultivated alga had an additional NADP+‐specific GDH (NADP‐GDH). These results suggest that NADP‐GDH compensates for the thermally induced decreases in nitrogen assimilation efficiency and thereby regulates nitrogen metabolism under conditions of temperature stress.