Studies in 40 liter aquaria indicated a preference by Tilapia zillii (Gervais) for the aquatic weeds Hydrilla verticillata Royle and Potamogeton pectinatus L., especially when fed in combination. Myriophyllum spicatum var. exalbescens Jepson was generally avoided and nutritionally inferior. However, in irrigation canals of southeastern California, the growth of M. spicatum was effectively reduced when T. zillii were present. Alternate nutritionally suitable food sources such as aquatic snails, crustaceans, and insects inhabiting the M. spicatum weed mass in canals could account for the effective feeding and significant reduction of this weed. Extensive reduction of H. verticillata in the All-American Canal by T. zillii feeding was also apparent.