Surveys were conducted in Florida, Louisiana, Panama, and Puerto Rico to determine the nature and extent of diseases occurring on water hyacinth (Eichhornia crassipes). Approximately 400 microbial isolates were inoculated onto water hyacinth leaves and seven fungi produced significant lesions. Rhizoctonia solani was the most virulaent pathogen isolated and it caused extensive lesions on aerial parts of the plant. Since it did not appear to be affecting other vegetation in those areas surveyed, its potential as a biocontrol agent should be further investigated. Cephalosporium zonatum was a less virulent pathogen but had a wide host range under artificial inoculation. It was shown to be identical to C. eichorniae and C. fici on the basis of culture morphology and pathogenicity. Fusarium roseum was shown to produce metabolites causing vascular browning in water hyacinth leaves, but it proved to be a weak pathogen. Nigrospora sphaerica, Apiocarpella sp., Curvularia lunata, and Pestalotia sp. were minor pathogens which had not been previously reported on water hyacinth. A lethal disease of anchoring hyacinth (E. azurea) was found in Panama but was not identified.