During the spring and fall of 1994 and winter of 1995, the exposure time of periphyton biomass on terval. In the streams with low periphyton biomass (chi. a: 2-4 mg/$m^2$) in natural rocks, biomass of arttificial substrata (unglazed tile: $3.7{ imes}9.5{ imes}2 cm$) exceeded that of the natural rocks after 28 days, while sites with high biomass (chi. a: 20-60 mg/$m^2$) in natural rocks showed slower biomass accumulation after 40 days. Due to the high licht input and temperature in a Partially shaded mountain stream, development of periphyton biomass in spring occurred faster than that of winter. In general, development of periphyton biomass placed on artificial substrata took 4-5 weeks in spring and at least 6 weeks In winter to reach the natural level.