Previous studies have shown that replication in vitro of the porcine parvovirus (PPV) isolate, KBSH, was restricted at 39°C but not at 37°C. In contrast, replication of the Kresse isolate was restricted at 37°C but not at 39°C. In this study, Kresse and KBSH isolates were passaged up to ten times in swine testicle (ST) cells at non-permissive temperatures, and at subsequent passage viral protein synthesis, viral DNA synthesis, and progeny virus were evaluated. KBSH became adapted for replication at 39°C upon serial passages, displaying an appreciable increase in viral progeny, viral polypeptides, and viral DNA concentration. This finding was also observed with Kresse virus isolate continuously passaged at 37°C. Neither isolate became adapted for replication at 32°C. In an attempt to examine the effect of in vitro passage at non-permissive temperatures on pathogenicity in swine, KBSH passaged 10 times either at 37°C or 39°C was inoculated into swine fetuses. Two of four fetuses inoculated with 39°C-passaged KBSH were dead and hemorrhagic or mummified. All four fetuses inoculated with 39°C-KBSH contained viral antigen and viral DNA. In contrast, fetuses inoculated with 37°C-passaged KBSH, or with cell culture fluid were normal in appearance. Viral antigen and viral DNA were not demonstrated in fetuses inoculated with 37°C-KBSH or cell culture fluids. These findings suggest the possibility that the ability to replicate at 39°C is associated with virulence in swine fetuses.