As for the electron-microscopic observation of the bacterial capsule, we can see only the reports by Mudd et al. (1943) on that of Diplococcus pneumoniae and by Labaw et al. (1954) on that of Escherichia coli. In this report. the author tried the electron-microscopic observation of the capsule of Klebsiella pneumoniae, and obtained the following results: 1) For the electron-microscopic observation of the bacterial capsule, the most important is the preparation of the materials; mount and dry bacteria without washing to avoid falling off of the capsular material, and, for this purpose, lyophilization is also a good method. 2) The capsular material is observed as two layers, the internal electron-transparent and external electron-semitransparent layers around the bacterial cell 3) In a young culture, the transparent layer is wide and the semitransparent layer is very narrow. With the lapse of culture time, however, the semitransparent layer gets wider, and, in a very old culture, the transparent layer disappears. 4) After the passage through the mouse, as compared with that of cultured one, the transparent layer appears very widery, and this is considered to be due to the increased secretion of the transparent capsular material. 5) The transparent material coagulate into gel with the lapse of time; this can be well observed after heating. 6) The semitransparent material seems to be nothing but the viscous substance in the colonies of capsule-forming bacteria. These materials can be observed only in capsule-forming bacteria and completely accord with the capsule with is observed light-microscopically in stained preparation. From these results, the author believes that these materials are the bacterial capsule itself and that the capsule could be photographed electron-microscopically.