An in vivo-passaged strain of Mycoplasma hyopneumoniae attained viability titers of 10(6) to 10(8) color-changing units per mg of tissue in pig lungs and caused gross and histological pulmonic lesions. Mycoplasmas were readily located in the lumina of the respiratory tract by electron microscopy. In sections of tissue fixed in glutaraldehyde-osmium, the organisms were found to possess many radial fibrils on the outer surface of the limiting membrane. These fibrils appeared to interconnect adjacent mycoplasmas and to extend between the organism and epithelial cell. Ruthenium red staining demonstrated a thick, dark layer of capsular material enveloping the entire mycoplasma cell. The capsular material was seen to bridge the space between the mycoplasma and host cell. The general morphology of the in vitro-passaged strains grown in broth medium was essentially similar to that of the in vivo-passaged strain. In these organisms, however, no long fibrils were seen, although a fuzzy layer was present outside the cell membrane. The ruthenium red-positive capsule was stained less intensely, and its width was only about one-half that observed for the in vivo-passaged strain. In negatively stained preparations, the cells had an outer fringe of amorphous material apparently corresponding to the fuzzy layer seen in thin sections. The in vitro-passaged strain grew poorly in pig lungs and lost its ability to produce gross pulmonic lesions. The organisms in the respiratory tract had a capsule much thinner than that of the in vivo-passaged strain.