Fungi were isolated from diseased and decaying water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub. from twenty three sites in eastern Australia. Some of the fungi had not previously been recorded on this host, but the majority were species known on water hyacinth in America and Asia, including pathogens Acremonium zona tum (Sawada) Gams and Cercospora piaropi Tharp. Bacteria isolated were Pseudomonas fluorescens and a non-fluorescent pseudomonad unlike any published species description. A greater variety of fungi occurred on plants damaged by the insect biological control agents, Ncochetina eichhorniae Warner and Sameodes a/bigullalis (Warren) than on plants not attacked by these insects. Many of the species are ubiquitous on decaying plant material. No particular fungus could be recognised as the main cause of necrosis which is associated with damage by insect larvae to the petiole and rhizome, but microorganisms are considered to be important in achieving the overall biological control. In contrast to other infections of the lamina, the lethal effect of a leaf blight was linked to severe infection of the lamina of the younger leaves. In glasshouse pathogenicity tests, Fusarium spp., A. zona tum and C. piaropi caused more disease than any other microorganism tested. A. zona tum and C. piaropi are considered to have the greatest potential for development as mycoherbicides.