Abstract: On 16 May 2012, a herd of 75 dairy cows in southwestern France had hyperthermia and decreased milk. Of 18 cows tested 9 were positive for Schmallenberg virus (SBV) by PCR and negative for SBV by ELISA, one was positive by PCR and ELISA, and 8 were negative by PCR and ELISA. On May 23, a week after the first samples were collected, all cows tested were positive by ELISA and only 1/18 cows were positive by RT-PCR. On 29 June, all 18 cows tested were negative by RT-PCR. Detection of SBV-neutralizing antibodies by virus neutralization assay in serum samples obtained on May 23 confirmed SBV ELISA results and showed that a commercial IgG ELISA is suitable for detection of acute cases of SBV. Accordingly, serologic and molecular data showed that acute SBV infection occurred in cattle in southwestern France in May 2012, suggesting that SBV overwintered or was reintroduced.