We report on a previously healthy 56-year-old woman who was referred to our hospital with fever and progressive left foot pain. She had been bitten by a cat 7 days previously, and cephalosporins had been prescribed for treatment. However, her clinical symptoms deteriorated, and physical examination on admission was compatible with necrotizing fasciitis. Treatment with ampicillin-sulbactam and clindamycin was initiated. In addition, immediate surgical debridement was performed, resulting in therapeutic success. Culture of the necrotizing tissue grew multiple organisms, including Pasteurella multocida and Bacteroides caccae. Administration of appropriate antibiotics after a cat bite is essential for the prevention of potentially fatal complications.