A survey of the fungal and bacterial flora of waterhyacinth, [Eichornia crassipes (Mart.) Solms] infested with two arthropods, Neochetina eichhorniae Warner (weevil) and Orthogalumna terebrantis Wallwork (mite) was undertaken to determine the role of microorganisms in the decline of this host in Florida waters. Generally arthropod-infested plants were more diseased, had smaller laminae, lower numbers of live petioles and higher numbers of dead petioles per plant than noninfested plants. Several parasitic fungi, including some known pathogens of waterhyacinth, and soft-rot bacteria were isolated from arthropod-infested and noninfested plants. Higher numbers of microorganisms were present on arthropod-infested than on noninfested plants. The fungi and bacteria occurred randomly on arthropod-damaged waterhyacinth. No vector relationship was discovered between the arthropods and any fungus or bacterium. However, Acremonium zonatum (Saw.) Gams a virulent pathogen of waterhyacinth, and other necrotic leaf-spot diseases (‘disease B’) were predominant on arthropod-damaged plants. Disease B was independent of the arthropods, while A. zonatum incidence on waterhyacinth in sample sites was related especially to damage by adult mites. The origin of the observed association between A. zonatum symptoms and mite damage on laminae is unclear, even though field observations suggest a direct relationship between them. The weevil was also capable of aiding A. zonatum infections by creating points of entry for this pathogen into the host. In laboratory tests, waterhyacinths could be killed by the combined effects of the weevil and A. zonatum, although death did not ensue in the field under existing levels of this disease. The combined effects of arthropods and diseases led to reduction in the photosynthetic area of waterhyacinth and under severe stress, to root- and crown-rots. It is recommended that the process of biological control of waterhyacinth in Florida should include an integrated approach, using insects and pathogens.