A practical method of continuous measurement of radon in groundwater for earthquake prediction, especially suitable for field observation is described. The detector consists of a ZnS scintillation chamber for alpha counting and a separation chamber in which radon is emanated from groundwater to the gaseous phase. The radon concentration in the gaseous phase equilibrates with that in water according to the distribution coefficient. Alpha activities of radon and its daughters (218Po and 214Po) in the scintillation chamber are counted and recorded continuously. The counting rate of about 1000 cpm/10-10 Ci Rn/1 is obtained by the detector, which has an effective volume of 1.6 1.