Ampullarium canaliculatus were collected monthly from the field in five areas in Pintung County and examined for larvae of Angiostrongylus cantonensis. Snails from one area showed the highest positive rate (30.8%) and the highest number (31) of larvae per snail. Snails were placed in four groups according to size. The positive rates in snails increased from 10.0 to 29.4% and the average number of larvae per snail increased from 11 to 40 as the size of snails increased from 1.0 to 4.0 cm height or more. In the seasonal fluctuation of snails in the field, positive snails could be examined every month; however, the positive rate in snails (28.2%) and the average number of larvae per snail (33) in the dry season (November to April) were significantly higher than those (15.2% and 23 respectively) in the rainy season (May to October). Ampullarium canaliculatus were confirmed naturally infected with A. cantonensis in this investigation so more attention should be paid to the snails to prevent the infection in humans.