The electron microscopic appearance of the cell surface of Bacteroides strains and Klebsiella pneumoniae stained with ruthenium red or colloidal iron is described. The effect of polymyxin B (PMB) was also registered. It was found that all Bacteroides strains have a polysaccharide lined 'micro-capsule' external to the outer membrane which could aggregate and form blebs. The blebs so formed were distinct from other types of bleb formed in Klebsiella involving the outer membrane and induced by PMB. Such types of PMB alterations were not induced in Bacteroides.