Tyzzer's disease was diagnosed as the cause of mortality in free-ranging hatch-year passerine birds in Southern California, US. Birds presented with ataxia, tremors, and opisthotonos and died or were euthanized. Postmortem examination of four birds identified encephalitis on microscopy with characteristic stacked, slender bacterial rods visible with special stains. Infection with Clostridium piliforme was confirmed by real-time PCR.