The eukaryote/prokaryote ratio permits immediate and direct estimation of ecological stability in closed microbial ecosystems as well as in natural systems. Previous research demonstrated partial pressure of oxygen can be used as a long term indicator of stability (Kearns and Folsome , 1981). The eukaryote/prokaryote ratio correlates with oxygen measurements and is another useful determinant in the studies of closed ecological systems. In the future this ratio may find widespread application since it is based on reliable internal properties of the system.