Described in 1926 from Florida and Mississippi, the phyline plant bug Keltonia balli (Knight) has been known in the literature only from 10 specimens; six additional specimens, reported herein, have been found in museum collections. The mirid's host-plant relationships have remained unknown. From 2002 to 2008, I collected 359 adults and 78 nymphs (instars II–V) on Florida rosemary (Ceratiola ericoides Michx.; Ericaceae), a dioecious evergreen shrub endemic to the southeastern United States. Nymphs and all but four adults were on male plants with flower buds or flowers in October, November, or December; the mirid was not found during sampling in January, February, March, April, June, or August. Keltonia balli was found only in the Lake Wales Ridge and other sand scrubs of peninsular Florida from Clay County south to Highlands County. The plant bug is a rosemary specialist whose nymphs develop on reproductive structures of male plants. Aspects of this herbivore-plant system needing elucidation, such as the number of generations of K. balli and factors that might synchronize its seasonality with a transient resource, are discussed.