This study was conducted to characterize periphyton biomass, productivity, and algal species composition, and to examine how these parameters changed seasonally over the course of two years in Walker Branch, a temperate forest stream. Seasonal variations in several factors potentially controlling periphyton (irradiance, streamwater nutrient concentrations, temperature, and the incidence of severe storms) were large. However, there was no consistent seasonal variation in herbivore (snail) density, which was high (> 1000 animals/m2). Regression analysis was conducted to determine the influence of potential controlling factors on periphyton biomass and productivity. Seasonal changes in periphyton biomass were small and only weakly related to irradiance (-), snail density (-), and temperature (+). Biomass was lowest in early spring and highest in summer in 1989; but during the following year, biomass was similar year-round. Seasonal variation in primary productivity was also relatively small, but was positively related to inorganic nitrogen concentration and was highest during summer. Although there were some small seasonal changes in algal species composition, more striking was the fact that Stigeoclonium sp. (primarily grazer-resistant basal cells) dominated year-round, forming >45% of total algal biovolume. The lack of strong seasonal variation in periphyton biomass and productivity and the observed dominance by a grazer-resistant alga appeared to be primarily the result of the high and relatively constant density of snails. The lack of a relationship between periphyton biomass and productivity also suggested an overriding effect of snails, which can consume increases in productivity when they occur. These findings suggest that in streams where herbivore density is high, periphyton productivity and biomass may remain relatively constant, despite seasonal fluctuations in potentially limiting physical and chemical variables. Although high densities of snails were probably the primary cause of the lack of large seasonal variation in periphyton biomass, productivity, and species composition, strong asynchrony in several growth-limiting factors, such as nutrients and irradiance, may also have limited seasonal changes in periphyton, implying that multiple factors were important.