Abstract 1. 1. The blood osmotic pressure in the terrestrial isopod Porcellio scaber varies inversely with the acclimation temperature (as measured at 3, 23 and 30°C). This species also has decreasing osmotic pressures with increasing size. 2. 2. Between 23 and 30°C smaller animals did not show any difference in their blood oxmotic pressure, while in larger animals the blood concentration remained unchanged between 23 and 3°C. 3. 3. Moulting animals as well as ovigerous females have lowered osmotic pressures. There was no influence of sex on the osmotic value in intermoult animals. 4. 4. The blood concentration of Oniscus asellus was not related to body size when measured at 23°C. 5. 5. Some ecological and metabolic implications of these findings are discussed.