The knowledge of ascidian biodiversity especially that of the family Didemnidae is incomplete. Collections were made from intertidal rocky areas, a depth of 2-6 meters, trawl discards and mussel landing centres of the coastal districts - Ramanathapuram, Tuticorin and Kanniyakumari. Ten species of colonial ascidians of the family Didemnidae - six of the genus Didemnum; one each of Diplosoma, Lissoclinum and two of Trididemnum were identified and added to the records of Indian waters from the Gulf of Mannar region. All the species are described in detail with original camera lucida drawings, taxonomic keys and illustrated with photographs of colonies, spicules and larvae.