The pathogenicity of 5 different encephalomyocarditis (EMC) virus isolates was investigated in swine fetuses following injection of each virus in utero. Laparotomies were performed on 3 pregnant sows in early mid-third (39-40 days) of gestation and on 5 sows in the late mid-third (70-72 days) of gestation, and groups of fetuses were inoculated with different viruses into the amniotic sacs. The uninoculated fetuses served as controls. Thirty-five (71.4%) of 49 infected and 1 of 26 control fetuses were grossly abnormal. Virus was recovered from 18 of 28 infected fetuses and 1 of 16 control fetuses examined. Antibody to EMC virus was detected in all of 14 fetuses infected at 70-72 days of gestation and examined 11-26 days post-infection. The fetal pathogenicity was different depending on the virus strains and the fetal age at the time of virus infection. The EMC ATCC-VR 129 virus was not pathogenic but NVSL-PR, MN-25 and MN-30 were highly pathogenic to the fetuses in both early and late mid-thirds of gestation, while NVSL-MDV was pathogenic to the fetus in early but not in late mid-third of gestation. Possible mechanisms for differences in the pathogenicity between the virus strains are discussed.