A new species of leafmining fly in the genus Phytomyza Fallen (Diptera: Agromyzidae) is reared from Gelsemium Juss, representing the first known instance of an agromyzid feeding on a host plant in Gelsemiaceae (Asteridae). The host plant, likely G. sempervirens (L.) (the “evening trumpetflower”), but possibly also G. rankinii Small, is a perennial vining species native to the southeastern United States. All examined specimens of this leafminer were reared from leaves collected in North Carolina from January to April in 1996 and 1997. The morphology and life history of the new species, Phytomyza omlandi Scheffer and Lonsdale, are discussed; photographs and illustrations are provided for external structures, the male genitalia and the leafmine. Morphological and molecular data support a close phylogenetic relationship between this species and the holly leafmining P. ilicis Curtis complex, although exact relationships are still uncertain and studies are ongoing.