In 2002, mosaic symptoms associated with yellowish ringspots were observed on leaves of a hybrid of lobelia (Lobelia spp.) grown in a public garden in Alsace (France). In 2003, similar symptoms were observed in Lobelia laxiflora in the Hanbury botanical garden (La Mortola, Italy) and the botanical garden of Nice (France). Cucumber mosaic virus (CMV) was identified in samples collected from the three locations on the basis of the following: symptoms exhibited by a host range of inoculated plants previously described (1); the observation of isometric particles (approximately 30 nm) with an electron microscope in crude sap preparations from inoculated plants and semipurified extracts of Claytonia perfoliata; and the positive reaction in double-antibody sandwich enzyme-linked immunosorbent assays (DAS-ELISA) to antibodies raised against CMV (2). In double-immunodiffusion analysis, each isolate was shown to belong to the group II strains of CMV (4). In these experiments, no differences were observed among the isolates collected. To test if CMV was responsible for the symptoms observed in Lobelia spp., an isolate from Alsace was grown in Xanthinc tobacco plants following isolation from local lesions on Vigna unguiculata and then mechanically inoculated to L. × speciosa cv. Compliment mix (10 plants), L. siphilitica (10 plants), L. inflata (Indian tobacco) (10 plants), L. erinus cvs. Crystal and Empereur Guillaume (5 plants), L. erinus pendula cvs. Saphyr and Cascade (5 plants), L. laxiflora (10 plants), and L. × gerardii cv. Vedrariensis (5 plants) and grown in a hydroponic system. Eight weeks postinoculation, all plants except L. laxiflora exhibited systemic mosaic and chlorotic ringspot symptoms on leaves and resulted in strong DAS-ELISA reactions for CMV, whereas mock-inoculated controls remained symptomless and virus free. Symptoms were particularly severe on L. siphilitica and L. × speciosa, but mild on L. inflata and L. × gerardii. Foliar mosaic symptoms appeared only 6 months postinoculation in 7 of 10 inoculated L. laxiflora plants. Only these plants were CMV positive using DAS-ELISA. No symptoms were observed in flowers of any plants infected with CMV. CMV has been previously reported in other species of the family Lobeliaceae including L. cardinalis, L. erinus, L. gracilis, and L. tenuior following natural or experimental infection (3) but Koch's postulates were not completed. This study validates that CMV is responsible for mosaic diseases in Lobelia spp., and shows that hybrids from L. cardinalis such as L. × speciosa and L. × gerardii also are susceptible to CMV. Mosaic symptoms in L. siphilitica and L. × speciosa are particularly damaging to their ornamental quality. Moreover, perennial plants such as L. laxiflora can be sources of CMV contamination by aphid transmission.