This paper proposes an index of group (racial, ethnic, sexual, etc.) occupational inequalities which can be computed from standard group-by-group occupational distributions, and which can be interpreted substantively as the relative likelihood that a group member will respond successfully to a new high status job opportunity, given that such opportunities are encountered at random throughout society. That is, the index summarises group differences in occupational aspirations and in all forms of discrimination encountered in the job market, in terms of conditional probabilities. Numerical values of the index have this same interpretation regardless of the occupational opportunity structure underlying observed occupational distributions. Thus the index can be used to compare group inequalities at different points in time, regardless of how the overall occupational distribution changes (and also regardless of changes in the relative sizes of groups). The index is applied to the analysis of changing occupational inequalities among Canadian ethnic groups between 1951 and 1961, and also to changing unemployment rates among blacks and whites in the United States between 1961 and 1969.