A rapid method has been developed for the de termination of214Pb and214Bi in natural waters by alpha-spectrometric measurement of214Po. Well water was filtered on a membrane filter impregnated with manganese oxide and followed by direct alpha-spectrometric analysis. A prominent alpha peak was assigned to214Po, which should be supported by the longer lived parents214Pb and214Bi. The activity ratios between214Pb and214Bi adsorbed on the filter were determined by using the change of activity of214Po as a function of counting period after the filtration. The adsorbed yield of214Pb was also determined from the growth of the alpha-peak due to210Po from a210Pb tracer added in the original water. The results indicated that the amount of214Pb was in radioactive non-equilibrium state with the Rn-222 dissolved, while the activity ratios of214Bi/214Pb showed on increasing tendency with the storage period of the water.