The aims of this study were to investigate aspects of the aural microenvironment in dogs. The aural microflora, histological features of the aural integument, and biochemical components of cerumen from healthy canine ears and those with otitis externa were studied. Interactions between canine cerumen and one member of the resident aural flora, Malassezia pachydermatis were also investigated. Gram-positive, coagulase-negative cocci, and M.pachydermatis were the most common resident flora found in 52 healthy canine external ears. Microscopic examination of cytological smears from cerumen and microbiological culture indicated that these organisms were present in low numbers. Gram-positive coagulase positive staphylococci, Gram-negative rods, and M.pachydermatis were isolated most frequently from 27 canine ears affected by otitis externa. Eighty percent of these inflamed ears were associated with microbial overgrowth. The numbers of microorganisms found in cerumen cytological smears appear to be correlated to the growth density of microbial colonies on culture plates. In an anatomical survey of the external ear canal, 40 canine ears were examined. The average length of the cartilaginous part of these canals was 5.3 cm. The diameter at the most proximal end of the annular cartilage averaged 0.5 cm; at the proximal end of the auricular cartilage the mean diameter was 0.7 cm; at the distal extremity of the ear canal, the average diameter was 4.8 cm. Morphometric stereology was used to evaluate histological features of 28 clinically normal and 15 otitic canine ears at four anatomical levels. Marked variation was found in the distribution of sebaceous and apocrine glands in the aural integument in healthy ears and those with otitis.