When Wolffiella floridana, an aquatic angiosperm in the family, Lemnaceae, was grown in axenic culture under continuous light in E medium containing 1.0% sucrose and a micromolar amount of (14)C-labeled myo-inositol (MI), MI was taken up by the growing plants and converted to phytic acid. After 13 weeks in labeled medium during which time there was a 1000-fold increase in fresh weight, 30% of the (14)C was recovered in ethanol insoluble residue. Extraction of this residue with EDTA released 70% of the label into solution. Phytic acid, identified by paper electrophoresis, ion exchange chromatography, and hydrolysis with phytase, accounted for most of this radioactivity although some label was also found in pentaphosphate and lower phosphate esters of MI. Very little MI was converted to cell wall polysaccharides under the conditions used. Results of this study indicate that Wolffiella floridana is a convenient tissue for the study of phytic acid biosynthesis under laboratory conditions.Lemna gibba G3, grown under short day conditions in medium of the same composition as that used for W. floridana, also formed labeled phytic acid as well as other labeled lower phosphate esters of MI.