The ammonia and phosphorus removal efficiencies of the microalgae Chlorella vulgaris and Scenedesmus dimorphus, during biotreatment of secondary effluent from an agroindustrial wastewater of a dairy industry and pig farming, were evaluated. The microalgae were isolated from a wastewater stabilization pond near Santafe de Bogota, Colombia. Batch cultures were mane using both species in 4-l cylindrical glass bioreactors each containing 2 l of culture. Chlorella vulgaris was also cultivated on wastewater in a triangular bioreactor: Three 216-h experimental cycles were run for each microalga and in each bioreactor: In the cylindrical bioreactor; S. dimorphus was more efficient in removing ammonia than C. vulgaris. However; the final efficiency of both microalgae at the end of each cycle was similar: Both microalgae removed phosphorus from the wastewater to the same extent in a cylindrical bioreactor Using C. vulgaris, the triangular bioreactor was superior for removing ammonia and the cylindrical bioreactor was superior for removing phosphorus. This study shows the potential of using these microalgae to reduce the environmental pollution of heavily contaminated agroindustrial waters currently disposed of untreated into the water-ways and streams of tropical Colombia. (C) 1997 Elsevier Science Ltd.