A morbilliform rash lasting from one to three days became epidemic in Minnesota in 1957. It occurred alone or in various combinations with fever and aseptic meningitis; the meningitis also occurred alone in six patients. Each strain of virus isolated from patients was identified by appropriate neutralization tests, and acute and convalescent phase human serums were assayed for neutralizing antibodies. The pathogen was thus found to be a strain of the enteric cytopathogenic human orphan virus ECHO-9. It resembled the prototype ECHO-9 in its behavior in cultures of cells from monkey kidney, human amnion, and the HeLa strain of human carcinoma, but it possessed the additional properties of pathogenicity for newborn mice and for tissue cultures of human foreskin cells. In its pathogenicity for newborn mice it resembled certain European, English, and Canadian strains of ECHO-9. This virus is believed to have caused as many as 400,000 infections in the whole state.