Two isomers of betamethasone 9α-fluoro-11β,17α,21-trihydroxy-16β-methylpregna-1,4- diene-3,20-dione (1), were isolated from the mother liquor of a manufacturing process and were characterized by NMR spectroscopy. They were identified as 8α-fluoro-11β,17α,21-trihydroxy-16β-methyl-9β-pregna- 1,4-diene-3,20-dione (2) and 14β-fluoro-11β,17α,21-trihydroxy-16β-methyl-8α, 9β-pregna-1,4-diene-3,20-dione (3). The stereochemistries of 2 and 3 were assigned on the basis of proton-proton and proton-fluorine NOE experiments. The structure of the former was confirmed by x-ray crystallographic analysis.