The peach X-disease organism (XDO) [probably a mycoplasma] was transmitted during June, July and Aug. to peach [Prunus persica] and chokecherry indicator plants exposed for 5 wk periods beside X-diseased chokecherry (P. virginiana) in the field. Of 387 indicator plants exposed in 1974, 26% developed X-disease symptoms compared to 5% of 359 plants exposed in 1975 and 3% of 273 plants exposed in 1976. Transmission to indicator plants could not be directly correlated with the numbers of Paraphlepsius irroratus, Scaphytopius acutus and Colladonus clitellarius vectors captured on stickyboards at the exposure sites. P. irroratus accounted for 87% of 9986 specimens of X-disease vector species trapped in orchards during 3 yr and was common June-Nov. Of 331 P. irroratus leafhoppers, 37%, but only 26% of 150 S. acutus leafhoppers transmitted the XDO from diseased celery [Apium graveolns] to celery test plants in greenhouse transmission tests. P. irroratus leafhoppers (21), tested individually, transmitted the XDO on 24% of the days (counting from the day of their 1st transmission) they fed on celery test plants. Four of 15 Orientus ishidae (Mat.) and 8 of 44 Scaphoideus, tentatively identified as S. carinatus and as S. diutius or S. melanotus, transmitted the XDO to celery. These new vector species were not abundant in cultivated orchards. P. irroratus is considered the most important X-disease vector in Michigan [USA] because of its abundance and high transmission efficiency.