The nature of the photosynthetic pathways of Cyperaceae found in Japan were investigated on the basis of Kranz anatomy, the CO compensation concentration and previously reported data. Among 301 species (96% of all cyperaceous species recorded in the region), 58 species were classified as being C plants. These C species were scattered among the tribes Fimbristylideae, Lipocarpheae, Cypereae and Rhynchosporeae in the subfamily Cyperoideae. The genera Cyperus, Eleocharis and Rhynchospora included, in Japan, both C and C species within a single genus. Using these data, an analysis was made of the ecological characteristics and geographical distribution of the C and C species in Japan. Although cyperaceous species grow in markedly different environments, the majority were found in wet and aquatic areas (61%) or shaded areas, such as forest floors (20%). Most of the C species were also hygrophytes (58%) and forest-living species (25%), and C species growing in mesic and dry areas were relatively rare. The C species inhabited wet and aquatic (75%), mesic (13%) and dry areas (6%) and showed marked ecological characteristics with respect to soil-moisture conditions, unlike other C plants, although they were absent from shaded habitats. In order to determine the climatic factors that influence the relative floristic abundance of C and C members of the Cyperaceae in Japan, the ratios of number of C species to the total number of members of Cyperaceae (C percentage) in 16 representative locales were examined in terms of various climatic variables. There were strong positive correlations between the C percentage and temperature. Among the C groups of three subfamilies, there were different distributional trends for various temperature regimes. The C subfamily Caricoideae increased its relative contribution to the cyperaceous flora with a decrease in mean annual temperature, while the C subfamily Sclerioideae exhibited the opposite pattern. The C group of the subfamily Cyperoideae did not show any marked change in pattern along temperature gradients, unlike the two other C subfamilies, and seemed to be heterogeneous in terms of its response to temperature. The relationships between the C biochemical subtypes and ecological characteristics are also discussed.