The distribution of Trichoptera larvae Glossosoma, that construct dome shaped case of sand grain, in relation to current velocity was investigated at a riffle in the Hirose River, Japan. The larval density increased with increasing current velocity. In a laboratory experiment using water-filled glass bottles, oxygen consumption rates in “perforated cases”, in which gaps in the upper side of the dome case were left open, were significantly higher under lotic than lentic conditions; their oxygen consumption rates were also significantly higher than values from individuals in “closed cases”, in which the case gaps were filled with adhesive under both lotic and lentic conditions. We measured larval mortality under lentic and lotic conditions in the laboratory using an artificial channel. Mortality of larvae in perforated cases was significantly lower under lotic conditions than under lentic conditions; individuals in closed cases showed higher mortality under both lotic and lentic conditions. These results suggest that larvae prefer high current velocity because they can not facilitate sufficient gas exchange under lentic condition by lack of undulation behaviour, and that the numerous small gaps in larval case enhances the efficiency of passive gas exchange by stream flow. We also discuss the functional significance of glossosomatid portable case.