Metarotaliella tuvaluensis n. sp. is described from shallow lagoon sediments of Funafuti Atoll, western central Pacific Ocean, where it is found attached to Triloculina barnardi and, less commonly, to Quinqueloculina laevigata and Q. oblonga. Metarotaliella tuvaluensis attaches to the tests of partly-grown miliolid individuals; responses of the latter include attempts to overgrow or surround M. tuvaluensis, malformation of the chamber added after the attachment, and modification of the external test ornamentation. Growth of the test walls of the miliolid onto the M. tuvaluensis test holds the latter firmly in position and may eventually completely envelope it. Sexual reproduction of M. tuvaluensis is by plastogamy in place on the miliolid test, involving two or more gamonts of unequal size, and the miliolid probably survives this. The relationship may be a fortuitous one of the rotaliine simply attaching to an available substrate, or alternatively may be an example of endoparasitism. It is rare, affecting less than 3% of the adults of the miliolid species involved, and, even if harmful, is therefore unlikely to have a significant effect on the population dynamics of the miliolids.