The spread of the pathogenic Hydramoeba hydroxena through host populations of the freshwater hydra provides an ideal system for the experimental study of epizootic (epidemic) processes. Initially small infected green hydra populations were allowed to expand exponentially. Using an increasing temperature regime as an environmental variant, an attempt was made to isolate and quantify the main components of an epizootic threshold level below which an epizootic or host-pathogen equilibrium condition would exist. Host and pathogen growth rates were identical at approximately 14°C, with the hydramoeba rate becoming increasingly less below this temperature and increasingly greater above. Such an environmental condition (exemplified by a particular temperature) provides, therefore, a mechanism for host-pathogen equilibrium, and in particular may explain the maintenance of the hydramoebae reservoir (with corresponding host survival) during cold seasons. Above the temperature threshold of 14°C a host-density threshold characterized by host population number, time, and fraction of host population infected, was isolated. These parameters of the threshold level, except the time component, were independent of temperature (no difference between 15° and 20°C). The characterization of the epizootic threshold in this host-pathogen system was distinguished from that conceptualized by other epizootiological and epidemiological investigators.