The biogenesis of oxalic acid in Oxalis corniculata has been investigated. In O. corniculata the bulk of the oxalic acid is produced by CO2 fixation both in light and in darkness but the rate of its photosynthetic formation is much higher than in darkness. Several other plants some of which are known to accumulate oxalic acid e.g., Biophytum sensitivum, Averrhoa carambola, Impatiens balsamina, Amorphophallus campanulatus and Colocassia antiquorum also fix 14CO2 into oxalic acid photosynthetically within 1 min of exposure to the gas. In O. corniculata14C can be detected in oxalic acid within 5 sec and about half of the total 14C fixed in the 70% ethanol soluble fraction can be located in this compound after 5 min. This is accompanied by a decline of radioactivity in two compounds, the chromatographic behaviour and melting points of one of which and its DNP hydrazone are similar to those of an authentic sample of glyoxylic acid. When glyoxylate 1, 2-14C is incubated with Oxalis leaf homogenate it is converted to oxalate-14C. Glycolate is also metabolized to oxalate. The conversion of both glycolate and glyoxylate are favoured by light. The C2 compounds acetate and glycine however are utilized rather poorly. Sucrose-14C is also not metabolized markedly for this purpose.