The hypsoprorine treehopper Philya ferruginosa (Goding), described from Arizona in 1894, has remained little known. Images of Philya posted recently on the internet represent misidentifications of the Neotropical P. lowryi Plummer, known only from the type locality in south-central Mexico; the records refer to P. ferruginosa. The nymphs and adults of P. ferruginosa we found on New Mexico locust (Robinia neomexicana A. Gray; Fabaceae) in southeastern Arizona are the first host records for the treehopper. We differentiate P. ferruginosa from its native congener P. californensis (Goding) and the Mexican P. lowryi, describe the fifth-instar nymph of P. ferruginosa, and present new records from Madrean Sky Island localities in Arizona. We regard as incidental the collection of adults from plants that seem unlikely to support nymphal development.