Xylella fastidiosa, the causal agent of Pierce's disease (PD) of grape, was isolated from diseased grapevines grown in Kosova, Yugoslavia. The Kosova isolate was a rod-shaped bacterium which showed a typically rippled cell wall under electron microscopy. ELISA comparisons indicated that the Kosova isolate was closely related to the U.S. PD strains and to several other strains of X. fastidiosa. When DNA extracted from diseased grapevines collected from Kosova was used as template in PCR with primer sets specific for X. fastidiosa, a band of about 730 bp diagnostic for PD bacteria was detected. DNA from the isolated Kosova bacteria and the type strain of PD yielded the same length of DNA fragment in PCR assay. The Kosova isolate was inoculated into young healthy grapevines through the roots with negative pressure applied to the shoots. Typical scald and scorch symptoms appeared on the leaves of the inoculated grapevines 40-80 days after inoculation. The same bacteria were reisolated from these inoculated diseased plants and used to reinoculate young grapevines. The reinoculated grapevines produced the same symptoms, thereby fulfilling Koch's postulates. This is the first confirmation that PD of grapes occurs in Europe.