Recent developments have emphasized the need for an adequate reference level from which to assess the degree of noise-induced hearing loss. The purpose of the present study was to determine the threshold of hearing for pure tones on a large sample of subjects (N = 500) drawn from a population exposed to minimal levels of industrial noise. Four age groups were studied: 18 to 24, 26 to 32, 34 to 40, and 43 to 49 years, inclusive. Threshold measurements were made from 250 to 8000 cps on a Beltone audiometer provided with PDR-8 earphones calibrated in a National Bureau of Standards Coupler 9-A. The threshold data were analyzed by age groups, ear, and sex in terms of three types of distributions: (1) original distributions which contained the data for all subjects tested; (2) screened distributions which contained the data only for subjects who passed rigid screening criteria related to otological disorders and extent of noise exposure; and (3) truncated distributions which contained only data ±15 db from the mode of the screen distributions. The results of the study indicate that in general, only minor differences (less than 5 db) exist between ears, but that women have more sensitive and less variable hearing than men. The sex difference is independent of age and appears to be more marked at the higher frequencies. Although the effects of age are clearly evident for both men and women, there is limited agreement between the curves of presbycousis of the present study and those proposed by the American Standards Association. The suggestion is offered that future audiometric standards be specified independently for men and for women according to particular age levels. [This research was supported by the United States Air Force under Contract AF 33(616)-2626 monitored by the Bio-Acoustics Branch, Aero-Medical Laboratory, Wright Air Development Center, Ohio.]