The occurrence of Lophodermium on fallen needles and Leptostroma, the mitosporic state of Lophodermium, on living needles of Japanese black pine trees treated with simulated acid rain (SAR, pH 3) or tap water (control, pH 6·3) were studied. The amount of fallen needles was also examined. Significantly more needles fell from SAR-treated trees than from control trees, though there were no visible symptoms in the needles of SAR-treated trees. The detection frequency of Lophodermium on fallen needles, and that of endophytic Leptostroma on 2 year old and 3 year old needles, was lower in SAR-treated than control trees. The reduction in the detection of Lophodermium/Leptostroma on needles implied the reduction in the density of endophytic fungi with air pollution.