Abstract: To clarify the role ofwaterfront vegetationof floodplainsfor adult aquatic insects, Trichoptera and Diptera(Tipulidae and Chironomidae), in the middle reachesof the Chikuma River from May to July, an investigation of the number of these insectswas conducted by trapping in each type of vegetation using board traps. A total of 2608. 5 adults/m2were collected, andwe identified a total of 26 species belonging to three taxa, i. e., seven species of Trichoptera, four species of Tipulidae and 15 species of Chironomidae. The most abundant specieswas Psychomyia acutipennis in Trichoptera (95. 7%).There was a significant difference between the number of P. acutipennis in the allvegetation area(especially, Salix subfragilis)and the control area(no vegetation) during the investigationperiods(P0. 05, Mann -Whitney U -test). Other taxa didnot show a significant difference between the all vegetation area and the control. Moreover, the numbers of adult P.acutipennis showed a significant difference in height on each vegetation. In the case of Vicia villosa varia and V.villosa varia plus dead Phragmites australis, the highest number was caught in the traps set in the boundary between one plant and the plant above (P0. 05, Steel -Dwass Test)inMay. On the other hand, in the case of almost all vegetation during the investigation periods(except of S. subfragilis in May, Melilotus officinalis plus dead P. australis in June), the highest number was caught in the traps set up within the vegetation(P0. 05, Steel-Dwass Test). As a result, a significant difference was observed in the number of trapped P. acutipennis according to the vegetation and its height. It issuggestedthat the existence of multiple typesof vegetationin the floodplain plays an important role for maintaining the diversity of the fauna there.