The bacterial flora of chronic venous ulcers treated with an occlusive hydrocolloid dressing were studied over a period of 8 weeks. A novel exudate sampling method was used in an attempt to isolate anaerobic bacteria. The flora was generally stable. Once a species was present, it remained with the exception of Pseudomonas, which appeared to be inhibited by the dressing. Twelve out of 20 ulcers contained anaerobic bacteria and healing did not appear to be impaired by the presence of any particular species of bacteria.