A description of the first known mutualistic interaction between a cnidarian and a pelagic tunicate is presented. Calycopsid anthomedusan polyps, belonging in the genus Bythotiara, were discovered associated with individual zooids of a new doliopsid (Thaliacea: Doliolida: Doliopsidae) collected from the mesopelagic waters of Monterey Bay. One hundred and seventy five polyp-bearing doliopsid colonies have been observed and 43 were collected from 1995 to 2004. Colonies were observed in situ and then brought back to the laboratory and maintained in aquaria. More than 250 individual anthomedusan polyps were removed from positions at the buccal siphon of their doliopsid host zooids and maintained in the laboratory until they released medusae. The morphology and taxonomy of the polyps and medusae are described. A unique tentacle-based asexual reproduction strategy is discussed. The relationship between the doliopsid and the polyp appears to be mutualistic, with the doliolid obtaining both nutrition and protection from the anthomedusan polyps, while the polyps gain access to a mobile, food-rich substrate.