The effect of ammonia concentration on growth and physiology of a unicellular green alga, Chlorella vulgaris, was investigated. Growth occurred in all ammonia concentrations examined (10-1000 mg N l(-1)) although less growth was found in cultures containing either very low (10 mg N l(-1)) or very high (750 and 1000 mg N l(-1)) ammonia concentrations. At NH3-N concentrations between 20 and 250 mg N l(-1) there were no significant differences in specific growth rates and maximal cell densities attained. Growth in these media was comparable to growth in the commercial Bristol medium which contains nitrate as the nitrogen source. Higher chlorophyll and protein contents were found in cell cultures with higher ammonia concentrations. The algal growth was accompanied by a decrease in nitrogen content in the medium, indicating that nitrogen removal was due to algal uptake and assimilation. In cultures containing nitrogen lower than 40 mg N l(-1), nitrogen was completely removed at the end of the cultivation period Over 95% ammonium removal was found in cultures containing 40-80 mg N l(-1). The percentage N reduction decreased with the initial N concentrations in cultures containing more than 80 mg N l(-1). Copyright (C) 1996 Elsevier Science Ltd.