During the past few years in the course of large-scale laboratory rearing of Zootermopsis angusticollis Hagen, various diseases have repeatedly been observed to kill individuals or groups of termites. Occasionally these have taken on the proportion of an epizootic, but more frequently they have shown up sporadically in the death of one or a few individuals of a group. The mortality resulting from these diseases is a serious factor in the experiments being carried on upon the problem of caste differentiation in Zootermopsis by Dr. S. F. Light of the Zoology Department of the University of California. It was with the purpose of learning more concerning the causes of this mortality, that the present study was undertaken. Prom the practical viewpoint of laboratory control of the disease and from the biological viewpoint the problem is of considerable interest. Apparently the diseases studied are as yet undescribed for any termite. Aside from certain fungus diseases, the two bacterial diseases which are treated in this paper were the most noticeable causes of mortality in the laboratory stock. That they were singled out for study is due to the fact they were observed repeatedly and could be identified by their striking effect upon the termite. Undoubtedly with further work more disease-producing bacteria will be isolated from Zootermopsis.