The morphology of colonies of some pathogenic Gram-negative and Gram-positive bacteria has been studied by scanning and transmitted electron microscopy. The presence of covers on the surface of cells in colonies has been revealed. The examination of colony fragments in ultrathin section has revealed that cells exist in associations and the elements of cell covers are differentiated in the form of fibrillar structures in the intracellular space. This investigation has shown that covers in the colonies of the bacteria under study should be regarded as their morphological feature playing an important role in the development of the infectious process.