Abstract The potential effects of Beauveria bassiana, an entomopathogen, on the biological control agent of water hyacinth, Neochetina bruchi, were assessed. In bioassays, B. bassiana induced mortality of the eggs, larvae, and adults when the N. bruchi stages were either sprayed or dipped in conidia concentrations of 5 × 105, 5 × 106, 5 × 107, 5 × 108, and 5 × 109 conidia/ml. The fungus caused highest mortality in larvae, whereas eggs were most resistant. Mortality increased in all insect stages as the conidia concentrations increased. The LT50 values of the pathogen on eggs at 5 × 108 and 5 × 109 conidia/ml were 10 and 7.2 days, respectively. For larvae and adults, the LT50 values at 5 × 109 conidia/ml were 3.6 and 6 days, respectively. The LC50 values were significantly different for all three stages at day 12 posttreatment, with the values of 9.5 × 107, 7.6 × 105, and 8.9 × 106 conidia/ml for eggs, larvae, and adults, respectively. The prevalence of B. bassiana in adult N. bruchi was measured at three water bodies in Zimbabwe. The prevalence of B. bassiana was highest in February–March of 2000. At Lake Chivero, the maximum reached 38%, whereas it was much lower at Lake Kariba and at Kudzwe Dam. Differences in the prevalence of B. bassiana appeared to be coupled with herbicide sprays used for the control of the water hyacinth problem. The prevalence was highest at sites where herbicides were used regularly and lowest where no herbicides were used. The temperatures at Lake Chivero were lower than those at the two other sites and this difference may have influenced the level of B. bassiana prevalence in N. bruchi.