Abstract The pine wood nematode, Bursaphelenchus xylophilus, causes a serious disease of native pines in Japan. The nematode was recently identified as a pathogen in the United States, and pathologists have speculated that it may threaten forests here. Its ability to kill native North American pines growing in forests has not been established, but evidence suggests that it kills exotic pines (Scotch pine, Pinus sylvestris, and Japanese black pine, P. thunbergii) in this country. Insect vectors transmit the nematode to cut timber and dying trees during vector oviposition. Thus the nematode can be present in dying trees without being the primary cause of death. Transmission during oviposition may explain its recent association with stressed trees in the United States.