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1. The extent of spatial and; temporal variation, and the taxonomic and biochemical composition of epilithic biofilm were determined in a first-order, acidic stream in the Ashdown Forest, southern England.

2. The biofilm had an autotrophic component consisting of acidobiontic diatoms, small coccoid green algae, zygnematacean filamentous green algae, and a heterotrophic component of bacteria. Fungi were absent.

3. Diatoms were the dominant algal group in spring. These were succeeded by coccoid green algae in summer. Synchronous variation existed between algae and bacteria, suggesting a strong link between the two groups.

4. Quantitative community descriptors (algal and bacterial densities, chlorophyll, ashfree dry matter) and biochemical descriptors (protein, lipid and carbohydrate content) of biofilm presented a consistent temporal pattern in which biofilm biomass peaked in spring and early summer and decreased subsequently.

5. Stepwise multiple regression revealed that temporal variation in the quantity of biofilm coating substrata was primarily associated with photosynthetically active radiation. The quantity of biofilm coating the upper surfaces of substrata increased with substratum particle size (small stones, large stones and bedrock). Smothering of algae by fine particulate organic matter, proximal shading by bed sediments, and physical disturbance are proposed as mechanisms that brought about the pattern.