The waterhyacinth grasshopper,Cornops aquaticum(Bruner), was studied in Uruguay and Argentina to determine if it is sufficiently host specific to introduce into the United States as a control agent against waterhyacinth,Eichhornia crassipes(Mart.) SolmsLaubach. Under field conditions, onlyE. crassipes,E. azureaKunth, and rarelyPontederia cordataL. sustained damage, and only the former 2 species were acceptable for oviposition and egg development. Based on the high degree of feeding preference forEichhorniaspp. in the tests and the dependence of the insect on members of this genus for its development,C. aquaticumis considered sufficiently specific toEichhorniaspp. to be potentially useful to introduce for biological control ofE. crassipesin the United States.