Results are presented for a pilot-plant scale study of algal culture in the liquid phase of pig slurry under temperate climatic conditions (latitude 54° 26′ N, longitude 6°06′ W). The pilot-plant was operated as a continuous culture over a two-year period and biomass production and nutrient stripping were intensively monitored. The two culture tanks were of raceway design each containing 2200 litres with a surface area of 11·1m2. The inoculated alga was Chlorella vulgaris (CCAP strain No. 211/1e). Retention times in the culture were set by the rate of addition of slurry which was prepared by screening, flocculation and dilution 1:9 with water to reduce the suspended solids content to 0·2%. Short-term algal productivities, corrected for slurry solids input, were between 7·65 and 32·9 g DM m-2 day-1 at a retention time of 4·5 days. The mean longterm productivity was 18·1 g DM m-2 day-1. The results suggested a 153 day growing season with a total algal productivity of 25·65 t ha-1. Phosphorus removal ranged from 42 to 89% and that for nitrogen ranged from 54 to 98%, BOD5 removal was approximately 98%. Differences between laboratory and outdoor pilot-plant cluture work are discussed and the importance of light as a limiting factor is considered.