Radon (222Rn), the immediate decay product of radium, is a causative agent of lung cancer. The seasonal indoor radon concentration in houses with different flooring, walls and roofs has been measured in Rajasthan, India. The measurements were made in houses using LR-115 type II plastic track detectors over four successive three-month periods (winter, spring, summer and autumn). An effort has been made to find possible relationships of indoor radon levels with building construction materials and ventilation condition of dwellings.