Stomatosematidi are by far the smallest of the four supertribes of Cecidomyiinae (Diptera: Cecidomyiidae) containing 56 species. The life history of only one species has been known previously: adults of Stomatosema nemorum Kieffer were reared from larvae feeding on a mushroom, Lactarius sp. (Fungi: Russulaceae). Here we describe the larva and adults of a new species, Stomatosema gagnei Kolesik, that was found inducing pustulate leaf galls on Causonis trifolia (Vitaceae) plants in Cape York Peninsula, Australia. Stomatosema gagnei is the first Stomatosematidi with a described larva and the first known to cause a gall. The fact that a species of Stomatosematidi, a supertribe hypothesized to be the sister group to all remaining Cecidomyiinae, was found inducing a plant gall suggests that the first of the transitions from the ancestral fungus-feeding habit to plant-feeding occurred in Cecidomyiidae at the base of the Cecidomyiinae clade more than 100–110 million years ago, during the lower Cretaceous.