Etiological studies to determine the cause of decline and death of Pinus spp. in Delaware were initiated in 1980. The pinewood nematode, Bursaphelenchus xylophilus, was found to be the major canse of mortality in Japanese black pine (Pinus thunbergii). When inoculated into healthy 5-yr-old Japanese black pines, B. xylophilus produced typical decline symptoms observed in the field. The xylophilous fungi most often associated with declining trees, Rhizosphaera pini, Fusarium spp., and Pestalotia funerea, were not pathogenic to Japanese black pine in greenhouse tests. Mineral analyses of soil and foliage showed no significant differences between healthy and infested trees. B. xylolyhilus was also found on loblolly pine (P. taeda), scrub pine (P. virginiana), Scots pine (P. sylvestris), red pine (P. resinosa), Eastern white pine (P. strobus), and pitch pine (P. rigida).