From October to December, 1986, eleven California sea lions were admitted to the California Marine Mammal Center (CMMC) exhibiting similar symptoms of lethargy, anorexia, and tonic-clonic convulsions and opisthotonus. Two of the animals died upon admission, five died despite treatment, and four were treated and released. Selected viral, bacterial, and fungal cultures were negative. Serum or tissue testing for organophosphates, organochlorines, heavy metals, and lithium were within acceptable limits for pinnipeds. Tissue samples of the brain were negative for viral encephalitis and meningitis. Abnormal brain lesions were diagnosed as acute ischemic neuronal necrosis and multifocal poliomalacia consistent with possible anoxia and/or toxicity. All test results have been inconclusive as to the probably cause of this outbreak. Due to the lack of a definitive diagnosis, other possible tests and procedures will be discussed in the event of future outbreaks.