Water hyacinth (Eichhornia crassipes (Mort.) Solms) cover of 0, 5, 10, or 25% surface was established in fertilized ponds stocked with the fish, Tilapia aurea (Steindachner), at Auburn, Alabama. Measurements of water chemistry, phytoplankton density, and fish production were made during the 1973 growing season. Phytoplankton production was less in ponds with 10 and 25% cover by water hyacinth than in ponds with 0 and 5% cover. Competition of water hyacinth with phytoplankton involved shading and removal of phosphorus from the water. Concentrations of dissolved oxygen were lowest in ponds with 25% cover, but oxygen tensions in all ponds were adequate for survival and growth of fish. Reduction in phytoplankton growth in ponds with 10 and 25% cover resulted in much lower fish production. The presence of 5% cover by water hyacinth did not significantly affect fish production.