The F-lac episome was transferred from Escherichia coli to Pasteurella pseudotuberculosis. The parent P. pseudotuberculosis fermented glycerol, rhamnose, salicin, and xylose, produced urease, VW antigens, and glucose-6-phosphate dehydrogenase, was pigmented, pesticin-sensitive, and virulent, and had a calcium requirement at 37 deg C. Some of the P. pseudotuberculosis F-lac recipients were negative for all of these properties, but became positive for all of them again when the F-lac episome was lost. It appeared that the F-lac carried with it a genetic unit that could repress the expression of several seemingly unrelated properties, and that this repression was removed when the episome was lost.