The precursory role of avenic acid A (AVA) in the biosynthesis of the mugineic acid family (MAs) of phytosiderophores was studied by feeding ¹⁴C or ¹⁵N labeled compounds into iron-deficient oat roots (Avena sativa L. cv. Onward). Carbon-14 of methionine was incorporated into AVA and 2'-deoxymugineic acid (DMA) in the oat roots, while ¹⁴C of homoserine was not incorporated into either AVA or DMA. The molar radioactivity of DMA was higher than that of AVA. Incorporation of ¹⁵N into MAs was examined by feeding ¹⁵N-ammonium sulfate into oat roots. The value of ¹⁵N atom-% excess of DMA was higher than that of AVA.These results indicate that methionine, rather than homoserine, is the direct precursor of MAs in oat, which is similar to that in barley, and that AVA is not the precursor of the other MAs.