Phytopathogens have characteristics that make them desirable candidates as biological control agents for aquatic weeds: 1. numerous and highly diverse, 2. easily disseminated and self-maintaining, 3. often host-specific, 4. non-pathogenic to animals and 5. capable of limiting populations without eliminating the species. Phytopathogens have been little investigated for the biological control of water weeds. Recent efforts have revealed several diseases with the potential for exerting a degree of control over populations of water hyacinth (Eichhornia crassipes), hydrilla (Hydrilla verticillata) and alligator-weed, (Alternanthera philoxeroides). These include a blight and leafspot of water hyacinth caused byRhizoctonia solaniandCephalosporium zonatumrespectively, a root and crown rot of water hyacinth of unknown cause, a virus-induced stunt of alligator-weed, aPenicilliumlysogenic to hydrilla and a pythium-induced blight of hydrilla.