Several batch culture studies were carried out to evaluate an anaerobically treated effluent as a low-cost growth medium for the microalga Chlamydomonas acidophila and to study the effectiveness of the microalga in removing NH4-N from the effluent. An initial decrease in the effluent pH to 3 was required for adequate growth of C. acidophila and removal of NH4-N. Growth of the microalgae was inhibited at high light intensity (224 mu mol photons m(-2) s(-1) at the surface of the vessels). However, the growth was not greatly affected by the high solid content and turbidity of the effluent. The microalga was able to grow in media containing NH4-N at concentrations of up to 1000 mg L-1 (50% of effluent) and to remove 88 mg of NH4-N L-1 in 10 days. C. acidophila therefore appears a promising agent for the removal of NH4-N from anaerobically treated effluents.