Samples of Cladonia mitis were collected from three microhabitats within an open field in eastern Hokkaido, Japan. Two sets of sample thalli were taken in close proximity to geothermal vents and a third from a neighbouring area not directly affected by outgassings. Enzymes were extracted and separated by isoelectric focusing, and multiple enzyme forms of three oxidoreductases, glucose-6-phosphate dehydrogenase, glutamate dehydrogenase and superoxide dismutase, and two hydrolases, esterase and alkaline phosphatase, were examined. Similarities between microhabitats based on electrophoretic results were calculated using Jaccard's similarity coefficient and expressed graphically through the complementary techniques of clustering and ordination. All of the major differences in isozyme patterns between remote samples and those near fumaroles were in the two highly polymorphic enzymes, esterase and alkaline phosphatase. Differences were as great or greater than those between geographically separated conspecific populations of lichens examined previously. In the more sulphurous microhabitats, nine bands were much more frequent and eight much less so than in locations further from the source of gases.