A possible infection with encephalomyocarditis virus was investigated on two Minnesota pig farms which experienced an increase in stillborn and mummified fetuses, high pre-weaning mortality and reduced farrowing rates. The monthly averages for the numbers of piglets born dead per litter on farms A and B reached 4-6 and 3-6, the pre-weaning mortalities 50 per cent and 31 per cent, and the farrowing rates 52 per cent and 63 per cent, respectively. Serological and histopathological examinations supported a diagnosis of infection with encephalomyocarditis virus, but attempts to isolate the virus failed. Specific antibody to the virus was detected in both fetal and neonatal sera collected from abnormal litters. The predominant histopathological finding was myocarditis consisting of focal or diffuse mononuclear cell infiltration. The detection of specific antibody, and the myocardial lesions in stillborn fetuses, suggested that the problems were associated with infection by encephalomyocarditis virus.