During an examination of fishes for the parasitic crustaceans, fourteen specimens ofArgulusbelonging to two different species, viz.A. indicusWeber, 1892 (eight specimens) andA. siamensisWilson, 1926 (six specimens) were collected from the fish,Ophiocephalus maruliusHamilton, taken from a canal in the environs of the village Champahati, District 24 Parganas, West Bengal. Out of seven fishes so obtained only three were found to be infected with the copepod parasites. While examining these copepods, the attention of the writer was drawn to a viviparous monogenetic trematode infecting only one species ofArgulus, viz.A. indicus. It was rather unfortunate that the piscine host could not be examined at that stage for the helminthic infection. About one hundred specimens of this trematode were collected from the ventral surfaces of the copepods.