The host ranges of Cercospora piaropi and Acremonium zonatum, fungi native to Mexico and pathogens of waterhyacinth (Eichhornia crassipes), were evaluated using 31 plant species (some with several cultivars tested) representing 22 families of economic and ecological importance. The results showed that only waterlettuce (Pistia stratiotes), another abundant weed in Mexico, was infected by C. piaropi. The use of those pathogens in the biological control of waterhyacinth would not be expected to affect plants of economic and ecological importance in Mexico.