Adult crania from four human skeletal population samples were radiographed in order to study variation of the air cell system in the mastoid process. Six types of processes were recognized, i.e., sclerotic, diploic, diplo-pneumatic, pneumatic, and highly pneumatic. Frequency of non-pneumatized (acellular) processes in Eskimos was significantly lower than that found in the other three populations. Female crania in each sample showed fewer acellular processes than male, but the differences were statistically significant only in the Whites, Eskimos, and total sex samples. While present in each population, frequency of asymmetry of type was significant in only Whites and Eskimos. These results provide some additional evidence that a genetic factor is operant in determining the degree of cellularity of mastoid processes.