The division of labor in formal organizations has important consequences for the distribution of opportunities and rewards. This paper examines variations in job title structures across work roles. Analyzing 3,173 job titles in the California civil service system in 1985, we investigate how and why lines of work vary in the proliferation of job categories that differentiate ranks, functions, or particular organizational locations. The statistical analysis underscores the importance of three socialforces shaping the division of labor: ascription by race and sex; the power and social standing of occupational groups, especially the professions; and organizational processes of rationalization. Some implications of these results for studies of organizations and social inequality are discussed. Remarkably little attention has been paid to the structure of job titles, despite the sociological importance of the topic. Distinctions among job titles are clearly relevant to social stratification, since wages, promotion opportunities, and other perquisites are often attached to jobs. From an organizational perspective, job definitions are central to the study of organizational structure. We examine the structure of job titles within a contemporary bureaucratic setting: California state government. In particular, we investigate variations in the proliferation of job titles across different kinds of work, asking why some work roles are subdivided into many job titles and others into very few.