Mice of the NCS and NCS-D colonies, bred at the Rockefeller University, harbored in their intestine an endemic strain of slow lactose-fermenting Escherichia coli 081±: :H21 serotype. In addition, NCS mice have recently acquired E. coli 0109±:K48:H14. Both strains persisted during the period of observation, whereas they were not encountered in the feces of mice from two other colonies. Other coliform strains encountered were more transient in their occurrence. Since strains of E. coli 081±: :H21 and 0109±:K48:H14 are extremely uncommon in human beings, it seems probable that they possess specificity for the mouse host.