By enrichment culture and conventional microbiological techniques the dominant alga of sewage trickle filters was obtained in axenic culture and identified as Neospongiococcutn ovatum Deason. A wide range of carbon compounds failed to support heterotrophic growth or alter the mean generation time or final cell yield in the light. The rate of photosynthetic carbon dioxide fixation was also unaffected by the growth conditions. The enzymes of the tricarboxylic acid cycle were present at comparable specific activity in cell extracts, but significant uptake of [14C]-acetate or [14C]-glycollate was not detected. It is suggested that impermeability to exogenous compounds may be an advantage in a toxic environment.