The benthic centric diatom Melosira nummuloides dominated sites of heavy pollution on the Clyde Estuary, occurring in conditions conducive to heterotrophic growth—positive correlation with BOD and suspended solids and negative correlation against light penetration of the water. A clone (Renf-1) of the organism, isolated from a polluted site and rendered axenic grew in culture through a range of light intensities, with compensating light below 150 lx. Some initial growth occurred in the dark on medium enriched with site water. Growth was best at a salinity of 10‰. for samples adapted to 4, 10 and 12‰.. Addition of certain amino acids to the medium caused significant enhancement of growth, due to leucine, arginine and serine at 0 klx, leucine, arginine and glutamic acid at 0·1 klx and glutamic acid at 3·3 klx. Eight amino acids were accumulated by carrier mediated methods in the dark. Kinetics for arginine uptake were best (low K _m) in the dark and at a salinity of 10‰, with a corresponding greater growth enhancement. The significance of heterotrophic nutrition in affecting the distribution of the organism is discussed.