1. 25 fluorescent lamps were set along the open water duct (Flgs. 1,2) of the Uji Water Power Plant. This duct, followed by the blind duct (water tunnel), conducts the water from the Nango Dam (Seta-gawa) to the Uji Power Plant. The duct (the open part as well as the blind part) is inhabited by a number of net-spinning caddis-fly larvae, the nets and cases of which resist the water flow, so that the efficiency of the power plant is reduced, generally by 10 per cent or more. 2. The caddis-flies caught at the lamps in the night of July 5〜6 1955 were studied. Samples were taken from No. 1 lamp (sample A) and from No. 25 lamp (sample B) ; the former is situated at the top of the duct, i.e. nearest to the Seta-gawa, the latter is at about 650 meters distance from No.1 lamp and at the entrance of the tunnel. Only 1/30 of each sample was calculated, as is shown in Table 1. 3. Hydropsyche nakaharai, which in its larval stage is the principal inhabitant of the duct, occupies 51.4 per cent of sample A and 93.6 per cent of sample B of the total caddis-flies. 4. Number of ♀ caught at light is far greater than that of ♂, especially in the species, Hydropsyche nakaharai and Macronema radiatum. This result is based upon the different strengths of phototropism in both sexes, because the true sex-ratio examined on the pupae in the water was almost 1 : 1. 5. Of 220 Hydropsyche nakaharai-females caught at light 189 (85.9%) had egg-masses; of 210 Macronema radiatum-females 161 (76.7%) had egg-masses. 6. Sample A contained more such caddis-flies in percentage, that emerged from the Seta-gawa, than sample B.