Parasites of the genus Mammomonogamus affect the respiratory tract of domestic animals. The present study was carried out to determine the presence of Mammomonogamus laryngeus infection and to analyze its lesions in infected buffaloes in Rio Grande do Sul, Brazil, between April and November 1999. The infection rate was 30.5%. In 32 infected buffaloes, the worm pairs collected per animal did not exceed 20. The microscopic diagnosis showed intense polypoid to intramucosal proliferation at the entrance to the pharynx, to which the parasites had adhered, with foci of multifocal hydropic degeneration of the epithelium or individual degeneration of epithelial cells with mild intraepithelial inflammatory infiltrate. The submucosa revealed intense lymphocyte infiltrate extending into the salivary glands. The submucosa also showed formation of structures that resemble lymphoid follicles.