The structure of rubriflordilactone B (2) was determined by X-ray crystallography. However, the NMR data of the synthetic sample did not match those reported for 2. It was then suggested that the original sample contained an additional isomer of different solubility, pseudorubriflordilactone B (3), whose structure remained unknown. From theoretical calculations, reexamination of the NMR data, and biogenetic considerations, it is proposed that 3 should be the 16S,17R isomer of 2.