A brief summary of the literature exemplifying the indirect lines of evidence leading to the conclusion that ‐living ciliates conjugate in natural environments is followed by a description of the isolation of potential mates, conjugating pairs and recent exconjugants ofParamecium bur‐sariafrom a pond in Westwood Village. Calif. The data suggest that natural populations ofParamecium pursariapass through infrequent epidemics of conjugat