Wild hosts of the Pierce''s disease (PD) bacterium were identified in riparian weeds along the Napa River and near springs on Spring Mountain in Napa County, California, by enzyme-linked immunosorbent assay (ELISA). The Pd bacterium was found in field-collected Fragaria vesca var. californica, Montia linearis, Rubus procerus and Vinca minor. The bacterium that was cultured in vitro from R. procerus and V. minor produced typical symptoms of PD following inoculation of healthy grape indicator plants. Bacteria were not isolated in vitro from ELISA positive F. californica and M. linearis on any media tested. ELISA was used to identify PD bacterium in populations of different homopterans collected in widely scattered locations in California. The PD bacterium was found in wild populations of Carneocephala fulgida, Draeculacephala minerva, Graphocephala atropunctata, Helochara delta, Pagaronia tredecimpunctata and Philaenus spumarius. The PD bacterium was cultured in vitro from ELISA positive groups of C. fulgida, D. minerva, G. atropunctata and P. tredecimpunctata. The same groups of insects also transmitted PD to healthy Vitis vinifera ''Mission'' indicator plants. Using ELISA, it was possible to distinguish the PD bacterium in pure culture and in host tissues from other morphologically similar xylem-limited bacteria.