Oviposition sites and the number of larval instars of two mountain blackfly species, Simulium japonicum Matsumura and S. rufibasis Brunetti, were investigated. The eggs of both species, which were indistinguishable from each other by egg size, coexisted on bryophytes growing on a water-splashed boulder. They were laid individually without making egg masses, and most of them were found lower than 5cm above water level. Two eggs of S. japonicum were also found on a partially submerged fallen leaf. Nine larval instars were identified in both species based on the measurements of head capsules of laboratory-reared individuals. The growth rates calculated from the size of head capsules between successive two instars were almost constant, i. e., a mean of 1.27±0.082 (S. D.) and 1.25±0.045 for S. japonicum and S. rufibasis, respectively. The oviposition habit of these species was discussed in relation to the habitat conditions.