Bacterial meningoencephalitis most commonly affected lambs two to four weeks old (median three weeks, range three days to six months) with clinical signs of episcleral congestion, lack of suck reflex, weakness, altered gait and depression extending to stupor, but hyperaesthesia to auditory and tactile stimuli. Opisthotonos was observed during the agonal stages of the disease. Analysis of lumbosacral cerebrospinal fluid revealed a highly significant increase in protein concentration (P < 0.01) with a neutrophilic pleocytosis, but bacteriological culture yielded organisms in only a few cases. A response was achieved with high doses of dexamethasone and chloramphenicol in only one of 20 cases. Polyarthritis and liver abscesses in a number of lambs provided evidence of a previous bacteraemic or septicaemic episode but no definite source of the central nervous system infection was identified. In common with other infectious bacterial conditions which are prevalent during the early life of sheep, control measures should ensure an adequate transfer of passive antibody, repeated treatments of the navel, and hygienic conditions in the lambing and rearing environments.