Holland's (1973) theory served as a basis for designing a 15-hour career development seminar for 24 undergraduate students. A Solomon four-group design (Campbell and Stanley, 1963) was employed to evaluate the effectiveness of the seminar, with the Career Maturity Inventory (Crites, 1973) serving as the dependent measure. Seminar participants evidenced significant improvement over 22 control students on four of the six scales of the Career Maturity Inventory (CMI). It was evident that the use of a single theoretical approach to design career development seminars rather than an eclectic approach, had two major advantages. First, the use of Holland's theory provided the student an organized conceptual framework for viewing the world of work. Second, the use of the theory simplified the task of training career development group leaders by offering a unifying principle. The study demonstrated the effectiveness of the career development seminar in improving career maturity, and further attested to the validity of the CMI as a measurement and evaluation tool. (Author)