A survey of 222Rn levels in 453 houses of physics professors from 101 universities in 42 states (plus the District of Columbia) was carried out with 1-y exposures of nuclear track detectors, accompanied by an extensive questionnaire. The geometric mean concentration was 38 Bq/m3 (1.03 pCi/l), the standard deviation was times divided by 2.36, and the arithmetic average was 54 Bq/m3 (1.47 pCi/l). Correlations were studied with age of the house, environs, location of the detector in the house, number of floors in the house, what is beneath the house, integrity of the barrier between the house and the ground, wind conditions, draftiness, construction materials, ventilation, use of gas, and source of water. In general, these correlations were found to be much weaker than expected, indicating that geographical variations are the dominating effect.