This study compared the effects of interference competition in habitats of different complexity and in different densities. The influence of fish density and habitat structure was examined in manipulative experiments using young-of-the-year white spotted charr Salvelinus leucomaenis as a model species. The difference of specific growth rate (G) range, an index of interference competitive intensity, was significantly smaller in the structurally complex treatments than structurally simple treatments, while there were no significant difference between high-density and low-density treatments. Thus, physical habitat structure was more effective for mitigating interference competition than manipulating competitor density. Although interference competition was not affected by competitor density, mean G were suppressed in the high-density treatments. This implied that exploitative competition may cause the decrease of G rather than interference competition does in the high-density treatments. Mean G were also suppressed in the structurally complex treatments. Chaotic flow pattern created by physical habitat structures may decrease G by reducing foraging success of experimental fish in the complex treatments.