Roussea simplex is the sole member of the endemic family Rousseaceae from Mauritius. Once widespread and locally common on Mauritius, today R. simplex is critically endangered, with 85-90 known remaining individuals in a few scattered populations. We documented the unusual flowering and fruiting phenology and studied the pollination and seed dispersal ecology of R. simplex in the accessible flowering and fruiting populations. Endemic diurnal Phelsuma cepediana geckos were the only pollinators and the only animals eating the pulp and dispersing the tiny seeds. In experiments with captive geckos, we confirmed that geckos ingest the seeds and pass them apparently unharmed. This makes R. simplex one of the few known plants that use the same animal species for both pollination and seed dispersal. However, none of the seeds from fruits or gut-passed seeds germinated, highlighting the large gap that remains in our understanding of the germination and regeneration of R. simplex. Conservation management must address this in the near future to avoid extinction of this unique lineage, and we highlight several options for applied conservation.