Leaf scorch symptoms occurred in 12-20.6% of the pear trees (Pyrus pyrifolia) in the low-altitude areas of central Taiwan in 1990. Symptoms appeared on leaves in early July and stayed until the leaves dropped in winter. If unchecked, dieback of twigs and branches, and finally death of the infected trees occurred within a few years. The disease was closely associated with a xylem-limited bacterium. An electron micrograph showed that the bacterium was rod-shaped with rippled cell walls, and measured from 0.2 to 0.5 X 1.1 to 3.4 mum. The bacterium could be cultured on PD2 and PW media but not on general-purpose bacterial media. Colonies were convex, roundish, and creamy white with a smooth margin, and reached 0.1-0.2 mm in diameter after a 14-day incubation at 30 C. Serologically, our culture was not closely related to the Xylella fastidiosa MT1 strain originally isolated from tissues with alfalfa dwarf disease. Fifty-five percent of healthy scions grafted onto water shoots of the leaf scorch-infected trees showed leaf scorch symptoms, and 50% of greenhouse-grown seedlings artificially inoculated with a bacterial strain from scorched pear developed scorch symptoms. The bacterium was reisolated from those inoculated symptomatic tissues. Control seedlings inoculated with phosphate-buffered citrate-magnesium solution remained symptomless. Injection of oxytetracycline alleviated disease development.