Prosthiostomum trilineatum of the family Prosthiostomidae has been reported for the first time from Ratnagiri, the west coast of India. This species has broadly rounded anterior end, tapering body, distinct orange-yellow, black and white frontal transverse bands and yellow-black longitudinal stripe. For the first time histological details of this species are presented. This confirms the presence of a pair of free prostatic vesicles and sclerotized stylet peculiar to the genus Prosthiostomum. This species is known from Japan, Micronesia, Papua New Guinea, the Red Sea and Singapore. This is also a new record of the family Prosthiostomidae from this coast. To consider the currently known two colour morphs of this species, other than presented here, a comparative examination of the reproductive system and molecular details is needed.
