Species diversity is a mathematical expression of the internal variability of biotic communities. It is a relative measure of the degree of concentration of species within an assemblage. Diversities have been calculated for each of more than 400 death accumulation-samples of benthonic Foraminifera from the eastern Gulf of Mexico. Diversities were calculated for each sample, based on previously published population data, using the reciprocal of Simpson's (1949) modification of Yule's (1944) statistic, [EQUATION] where N is the total number of individuals counted, ni is the number of species of the i-th species, and K is the number of species. End_Page 614------------------------------ In recent years many writers have presented various measures dealing with the relative concentration or variety of species in biologic communities. The results of these studies suggest a high degree of order in community structure. This order is expressed by a regular arrangement of population elements into a definite hierarchal pattern characterized by dominants in association with a progressively diminishing number of subsidiary species. In populations of high diversity, differences between the numerical census of dominant and subsidiary species is low. Low-diversity populations have low variety, and census differences between dominants and subsidiaries are great. Various explanations have been proposed to account for these differences as well as the order in biotic communities. The responsible agents have been most frequently characterized as being the results of adjustments caused by intra- and interspecific competition, successional development, and trophic relationships. The writer believes that these processes are important ultimately only when the ecologically defining factors within the environment are limited to little variation. Foraminiferal diversity distributions in the eastern Gulf of Mexico are offered to support the contention that population complexity is primarily a function of variability in environmental conditions. Maximum diversity is confined to the continental slopes, and isodiversity contours from the edge of the shelf seaward follow bathymetric contours. Isodiversities on the continental shelf are variable and register bottom topography and the net effect of prevailing current and wave forces. Statistical error plotted for the calculated species diversities is below 10 per cent. Because population variety is limited by the degree of variability in the mechanical and non-mechanical ecologic factors characterizing the environment, diversity plotted in relation to depth alone shows no correlation. End_of_Article - Last_Page 615------------