SYNOPSIS.Coprozoic protozoa from elk, bison, bear, moose, coyote, marmot, cattle, horse, sheep and man were kept in feces at 4° C. for 2–6 months. During that time in most of the samples there developed the flagellatesCercomonassp.,Copromonas ruminantiumandMonas communis;the amoebaeVahlkampfiasp. andSappinia diploidea, and ciliates of theNyctotherustype, and an unidentified smaller species. There was a correlation between numbers of protozoa and bacteria. The same species of protozoa in soil or in soil mixed with boiled feces failed to live. Coprozoic protozoa may require certain essential metabolites from bacteria as do true parasites. The wide variation in appearance of cysts made it practically impossible to identify the protozoa with certainty in that stage. Reliance had to be on motile forms which readily developed in the cold cultur