Relationships between periophyton biomass (as ash-free dry mass; AFDM) and water velocity were determined in two rivers for communities under various stages of accrual. No difference in periphyton biomass could be detected maong low, medium and high velocity habitats for the earlystages of colonization. However, at the end of the colonization period there were differences, with a gradient from highest biomass at low velocities (<0.3 m/s) to lowest biomass at highest velocities (>0.7 m/s). In one river a changeover in this trend occurred during the exponential growth phase, with higher growth rates under higer velocities. Thus at the time of peak biomass (day 77) the highest biomass was recorded under high velocities and the lowest biomass under low velocities (p<0.05). After this point, sloughing occurred under high velocities, but the intermediate velocity communities continued to accrue and at the end of the sampling period these communities had the highest biomass. When the communities reached maturity (day 92) there was a unimodal distribution of biomass as a function of velocity with a peak in AFDM at 0.5-0.7 m/s. Contrary to these results, the periphyton biomass in the second river continued to be higher in the low velocity areas during the exponential growth phase. A new spate occurred before the peak biomass was reached in that river.