Secondary bacterial infection in skin lesions is a common problem. This review summarises a series of studies of the microbiology of several of these infections: scabies, psoriasis, poison ivy, atopic dermatitis, eczema herpeticum and kerion. Staphylococcus aureus and group A beta-haemolytic streptococci were the most prevalent aerobes and were isolated from all body sites. In contrast, organisms that reside in the mucous membranes close to the lesions predominated in infections next to these membranes. In this fashion, enteric gram-negative bacilli and Bacteroides spp. were found most often in buttock and leg lesions. The probable sources of these organisms are the rectum and vagina, where they normally reside. Group A beta-haemolytic streptococci, pigmented Prevotella and Porphyromonas spp. and Fusobacterium spp. were most commonly found in lesions of the head, face, neck and fingers. These organisms probably reached these sites from the oral cavity, where they are part of the normal flora. This review highlights the polymicrobial aerobic-anaerobic microbiology of secondarily infected skin lesions.