Morphological changes of chlorplasts in cucumber leaves inoculated with Sphaerotheca fuliginea were observed by means of an electron-microscope (Hitachi HU12-A). Both the infected and uninfected-tissues were fixed with 6.25% glutaraldehyde solution for 4 hours and then with 1% osmium tetraoxide solution for 4 hours. In chloroplasts of the uninfected tissues, chloroplast membrane (double membrane), developed granum stack (grana thylakoids) and storomal thylakoid were clearly observed. Starch grains and osmiophilic globules in stroma (Figs．3 and 4) became more large with aging (Figs. 5-8). However, large differences in the morphological change of choloroplasts were recognized between the uninfected and infected leaves. In the leaves of a week after inoculation, chloroplasts swelled and then contracted (Figs. 9 and 1O). Osmiophilic globules enlarged significantly two weeks after inoculation (Figs. 11 and 12). On the contrary, starch grains were not observed (Figs. 13 and 14). Furthermore, the irregular arrangement and the swelling of grana thylakoids were also occurred. As shown in Figs.13 and 14, at this stage，disintegration of chloroplast membranes were still not occurred (Fig. 14). Such morphological changes in chloroplasts were also recognized in infected-cucumber cotyledons (Figs.1 and 2).