This study describes the ultrastructure of Warthin-Starry (WS) stain-positive bacteria in abscess-forming reticular lymphadenitis (ARL) compatible with cat scratch disease (CSD). Sections containing WS-positive bacteria were re-embedded in Epon, and semithin sections were examined by electron microscopy. Silver particles were aggregated on the outer surfaces of the bacteria. Stereoscopic observations clearly showed that the bacteria were pleomorphic, rod-shaped, arranged in a row or at an angle, and frequently showed septal formation. Electron microscopy of ultrathin sections revealed that the cell wall possessed an outer membrane characteristic of gram-negative bacteria. The results indicate that rod-shaped bacteria with the WS-positive, gram-negative staining cause ARL histopathologically consistent with CSD.