Experiments were performed to study the individual and combined effects of current velocity and substratum composition on the waterlouse Asellus aquaticus (L.). Both factors affected growth, mortality, behavior, and food consumption of A. aquaticus. Short-term effects of increasing current velocity depended on the type of substratum. Critical current velocity for detachment was almost the same on sand as on a polished surface whereas on gravel A. aquaticus could withstand higher current velocities by hiding in interstitial spaces. Long-term experiments with different combinations of current velocity and substratum composition showed that current velocity had a greater effect than substratum on survival and growth. Substratum, however, had a greater influence on the distribution of individuals in the experimental units. Since growth was reduced at high current velocity and no changes in levels of food intake were observed it is concluded that a substantial amount of energy is required for withstanding current at higher flow rates. Furthermore, mortality showed a strong inverse correlation to growth. The interaction of effects of natural habitat factors may be better understood using an experimental and modeling approach focusing on energy budgets.