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 pathogensAcremonium zonatum(Sawada) Gams andCercospora piaropiTharp. Bacteria isolated werePseudomonas fluorescensand a non-fluorescent pseudomonad unlike any published species description. A greater variety of fungi occurred on plants damaged by the insect biological control agents,Neochetina eichhorniaeWarner andSameodes albiguttalis(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. zonatumandC. piaropicaused more disease than any other microorganism tested.A. zonatumandC. piaropiare considered to have the greatest potential for development as mycoherbicides.