Construct validity of the Thumin Test of Mental Dexterity was studied in two ways: (a) by relating test scores to level of formal education among 168 male job applicants and (b) by correlating test scores with WAIS IQs among 32 vocational counseling clients. Mental ability, as measured by the Thumin scale, increased significantly with increasing levels of education as did scores on the numerical and verbal comprehension subtests. In the second investigation, mental ability scores on the scale correlated .91 with WAIS IQs, and .92 with WAIS verbal scores of 32 clients for vocational counseling.