Populations of the waterhyacinth mite (Orthogalumna terebrantis Wallwork) of ca 200,000 galleries/m2 (or 10,000 per plant) caused serious damage to waterhyacinth [Eichhornia crassipes (Mart.) Solms.] in the field, but such damage was sporadic and confined to small areas of plants in midsummer. Two or three generations a year occurred in the field. Adults emerged in late December, March, and May and adults were found on waterlettuce (Pistia stratiotes L.) throughout the winter. Development, appearance of the different stages, behavior of immatures and adults, and manner of attacking waterhyacinth is described. Only small differences were found between the biology of the mites studied in Argentina and of that reported in Uruguay and the United States.