An aquatic fern, Azolla microphylla (strain 175 MI, Catholic University of Louvain, Belgium), a natural source of protein, was used in this study to produce low-cost feeds for the omnivorous-phytoplanktonophagous tilapia, Oreochromis niloticus L. Fish were grown in a recirculating system and fed with six different diets in triplicate groups. Diets were formulated with approximately similar total protein, ranging from 27.25 to 27.52% dry weight (dw), gross energy content ranging from 85.1 to 96.5 MJ kg-1 dw, and with different levels of dry meal Azolla (0, 15, 20 30 40, 45% diet dw). All diet levels with incorporated Azolla meal exhibited weight gain, thus it can be assumed that Azolla in good combination with local products can be used to promote fish culture development. The Azolla-free diet and the diet containing 15% Azolla produced the same growth performance. However, the least expensive diet containing 45% Azolla also exhibited growth and can be used as a complementary diet for tilapia raised in fertilized ponds. © 2004 Blackwell Verlag, Berlin.