One hundred and one species (in 36 families) of weeds on cultivated land in Taiwan were investigated for the occurrence of Kranz leaf anatomy and activities of key enzymes of C4 photosynthesis to determine their photosynthetic mechanisms. Based on the anatomical and biochemical analyses, 75 species were found to possess the C3 and 26 species the C4 pathway of photosynthetic CO2 fixation. Among the 26 C4 species, 15 species are in Gramineae, 6 in Cyperaceae, 2 each in Euphorbiaceae and Amaranthaceae, and 1 in Portulacaceae. Two C4 species in the Gramineae, namely Digitaria radicosa (Presl) Miq. and Sporobolus fertilis (Steud.) Clayton, were recorded as C4 plants for the first time. The biochemical subdivisions of these C4 weeds were also determined. As in the natural C4 populations, the NADP-malic enzyme subtype of C4 photosynthesis dominates the list of C4 weeds on this island (62%), while the PEP carboxykinase subtype is relatively rare (12%). NAD-malic enzyme subtype has an intermediate representation (26%). The high proportion of weeds in Taiwan being C3 plants is noteworthy, and it may be accounted for by the high precipitation in this subtropical island.