In a previous genetic study of a highly ampicillin resistant mutant ofEscherichia coliit was observed that certain classes of recombinants always showed smooth colony morphology. The present work was undertaken in order to investigate a possible correlation between ampicillin resistance and capsule production, giving smooth colony morphology. Two methods for preparing capsular material were compared: ethanol precipitation and negative pressure concentration. Chromatographic examination of these preparations indicated that the negative pressure concentration method gives a more native product with a larger molecular weight than was obtained by the ethanol precipitation procedure. We have compared the amount of capsular material produced by seven different strains, two rough and five smooth. Large variations were observed in the amount of capsule produced but there was no obvious correlation between capsule production and ampicillin resistance. The surface structure of five of the strains was studied by electron microscopy. The smooth strains showed two types of structures closely adhering to the outermost layer of the wall, small vesicles of the order of 40–150 mμ in diameter and minute lamellated units measuring from 80 to 400 Å in length. The rough strains showed very few of these structures. When capsule producing strains were grown at room temperature more capsular material was produced and more of these extra‐surface structures were observed. It is suggested that the small vesicles and minute lamellated units may represent cell wall material produced in excess am