The heat dissipation rates of the invasive amphipod Gammarus tigrinus Sexton, 1939 of the southern Baltic Sea were measured by means of direct calorimetry at the habitat salinity of 7% (males and females) and after gradual acclimation (only males) to lower 3%o) and higher (13%o and 19.5%o) values (T = 18 degrees C). The mean specific metabolic rate at 7%o amounted to 1.67 +/- 0.86 m Wg(-1) ww (n = 25, average wet weight 10.9 +/- 5.1 mg). Due to the sexual dimorphism and the smaller size of the females their specific metabolic rates were two-fold higher than that of males. Animals exposed to lower than habitat salinity insignificantly increased their specific metabolic rate by 42.5%. A reduction of 37.5% was observed when they were subjected to the highest examined salinity (19.5%o). G. tigrinus was thus able to change its metabolic rate by 56% in the studied salinity range from 3%o to 19.5%) (C) 2007 Elsevier B.V. All rights reserved.