The seasonal and geographic distribution of X-diseasedParaphlepsius irroratus(Say)was studied in south, central, and northwest Michigan in 1988 and 1989 using a DNA probe. C6c, a fragment of pWXl derived from infectedColladonus montanus(Van Duzee), detected eastern X-disease in diseasedP. irroratus. Maximum numbers of infected leafhoppers were detected at the beginning of the emergence of each generation in early June and in late September. In early summer the percentage of X-diseased leafhoppers at the various sites ranged from 12.2 to 43.3%, and in the late season from 3.3 to 36.4%. The X-diseased leafhoppers occurred only in south and central Michigan.