Biosynthetic studies have shown the incorporation of DL-[2,3,3-²H₃]-tyrosine into the chromophore of pseudobactin, a siderophore produced by Pseudomonas fluorescens B10. A subsequent feeding using DL-[2',5',6'-²H₃]-3,4-dihydroxyphenylalanine showed no incorporation of the label, suggesting that oxidation of the aryl ring occurs after the incorporation of tyrosine. The ¹³C NMR assignments of pseudobactin are also reported here.