The authors describe a case of a rare but important entity, Serratia marcescens cellulitis, which illustrates key features of this condition, including the high severity of infection, and an immunocompromised host. Management of this condition is challenging, with high mortality. The authors describe successful use of a quinolone where a carbapenem failed, despite testing sensitive on two occasions in vitro, and also demonstrate that surgery is not always required.We describe a case of bullous leg cellulitis secondary to Serratia marcescensin a patient with a complex background including well-controlled HIV and venous insufficiency secondary to multiple deep vein thrombosis. Despite appropriate antibiotic therapy with meropenem, her condition deteriorated, requiring intensive care unit support. She was reviewed multiple times by the surgical teams, who felt that surgical intervention was not warranted. After a switch in antibiotics to ciprofloxacin, she made a sustained and complete recovery.S. marcescenscellulitis is a rare entity, but certain clinical features are common to most reported cases, including an immunocompromised host, bullous lesions, and a severe illness, all of which are demonstrated here. Optimal antibiotic therapy is not established, and our experience suggests that perhaps quinolones are superior to carbapenems, for reasons which remain unclear. Surgery is an important consideration in S. marcescenscellulitis; however, our experience suggests that in selected cases, watchful waiting may be appropriate.