The biosynthesis of apiose was investigated in cell wall polysaccharide of Lemna gibba G3 (duckweed) and in detached leaves of Petroselinum crispum (parsley). Lemna grown either in short days or in continuous light incorporated 14C from a medium containing myo-inositol-2-14C into d-apiosyl and d-xylosyl units of cell wall polysaccharides. Labeled d-apiose was characterized by paper chromatography, by formation of labeled crystalline di-O-isopropylidene d-apiose, and by gas chromatography of trimethylsilyl derivatives of apiose and of its sodium borohydride reduction product, apiitol. Periodate oxidation of labeled apiose revealed 86 to 94% of the 14C was located in formaldehyde fragments corresponding to C3′ and C4. Comparison of this result with work reported by Grisebach and Doebereiner and by Beck and Kandler supports the conclusion that myo-inositol-2-14C was converted to d-apiose labeled specifically at C4.