in the late 1960s, melanoides tuberculatus snails were introduced in brazil from north/east africa and southeast asia. the first records of specimens infected with cercariae were registered in rio de janeiro state in 2001. the present study reports the occurrence of m. tuberculatus infected with larval trematodes in rio de janeiro city. bottom sediment was collected with dip nets and sieved through 0.25 inch-mesh screening. snails were transported to the laboratory in vials with stream water, then measured and individually isolated in glass vials with distilled water. they were exposed to artificial light and temperature to induce cercarial emergence. the most actively emerging cercariae were processed by differential staining and silver nitrate impregnation methods. negative snails were subsequently dissected. approximately 700 snails were collected. snail total lengths ranged from 1.2 to 3.3 cm. the prevalence rate was 15.76% although 53.76% of the snails were found infected in one of the sites. infected snails were infected with rediae and pleurolophocercous cercariae. cercarial morphology and chaetotaxy were consistent with those of the family heterophyidae mostly due to the presence of median dorsal and ventral fins on the tail and the absence of ci dorsal sensory receptors.