A total of 14 cerebrospinal fluid (CSF) samples from ruminants clinically suspected of suffering from listeric encephalitis were examined by polymerase chain reaction (PCR) for the detection of Listeria monocytogenes (L. m.). Of these samples, 11 were examined bacteriologically. Although the clinical diagnosis was confirmed in eight of 11 ruminants by histological and/or bacteriological examination of the brains, L. m. was only detected in one of the CSF samples using PCR, and in none by culture. The PCR-positive CSF sample was obtained from a sheep which had been treated with antibiotics prior to CSF sampling. From these findings, it was concluded that L. m. only occasionally gains access to the meningoventricular system in the course of listeric encephalitis of ruminants and that a reliable aetiological in vivo diagnosis of listeric encephalitis generally cannot be based on the detection of L. m. in the CSF of affected ruminants.