The effect of a dam on the life history of the dobsonflyProtohermes immaculatuswas examined by comparing two neighbouring streams with and without a dam on the small island Tokunoshima, southern Japan. Because the dam drains off surface water the temperature is increased downstream. The accumulated annual temperature above 12°C, the threshold for larval moulting, was 2400 degree‐days in the natural stream, but 3300 degree‐days downstream of the dam. Although the density of small prey (< 0.8 mm in width) was lower below the dam, large prey density (> 0.8 mm width) which is the important prey category for this size‐selective predator did not differ between the two streams. The larval period was 2 years in the natural stream. However, it was reduced to 1 year, and slightly smaller adults emerged, below the dam. Thus, temperature greatly affected the larval growth rate ofP. immaculatusand slightly altered their body size at maturity.