The infestation dynamics and histopathology of the freshwater leeches (Hirudinea: Piscicolidae) Myzobdella platensis and Myzobdella uruguayensis on teleost fish from eastern Uruguay, Hoplias malabaricus (Characiformes, Erythrinidae) and Rhamdia quelen (Siluriformes, Pimelodidae), respectively, were examined over a 2-yr period. Myzobdella uruguayensis infested the gill filaments with 100% prevalence and a mean intensity of 12.5. There was an aggregated distribution, infestation was most intense on the first 2 gill pairs, and leeches were usually attached to the ventral gill sector. The preference for internal or external gill filaments was independent of leech abundance. Hemorrhage with clot formation and fibrin deposition were observed at the attachment sites. Gill filaments also exhibited an inflammatory reaction with edema and infiltration of mononuclear leukocytes. Some gill lamellae showed telangiectasic changes. Myzobdella platensis was variably prevalent over years (1987, 71.4%; 1989, 37.5%) but with similar mean intensities (1987, 3.2; 1989, 3.5). Leeches were found mainly on the caudal fin rays; none was found on the dorsal fin. Skin at the site of attachment exhibited complete erosion of the epidermis and a massive cellular infiltration by mononuclear cells. The epidermis close to the attachment site exhibited basal hyperplasia and spongiosis. The dermis showed edema, and the presence of scarce inflammatory cells was verified.