The 1996 Chagos expedition collected 128 specimens of opisthobranch gastropods representing at least 50 species, of which only 41 species are described in this paper: two cephalaspideans both belonging to the genus
Chelidonura, two sacoglossan species of
Thuridilla, one sea hare with problematic taxonomy, and 36 species of nudibranchs. Of the nudibranchs, all the sub-orders are represented by one or two species only except the dorids, which comprise the majority of the collection. Two species are allocated to genus only, one species each of
Phyllidiopsis and
Dermatobranchus, while two species are similar to known species. Numerous species of Phyllidiidae were collected, but only 10 are confidently assigned to a taxon. The taxonomic identities of
Hoplodoris pustulata,
Dendrodoris tuberculosa,
Phyllidiopsis xishaensis and
Phyllidiella striata are defined and discussed. There are four new distribution records for the Indian Ocean and several species are of interest as they have been rarely recorded.