SYNOPSIS. To evaluate Vorticella populations in an activated sludge sewage‐disposal plant as a) indicators and b) contributors to purification, population behavior in the plant was compared with behavior in clonaI cultures maintained with mixed sludge microflora. In a 12‐month survey of protozoa in the plant, of 37 spp recorded only 9—all ciliates—were indigenous. Ciliate populations fluctuated cyclically. Alternations in dominance of Vorticella convallaria (L.) with V. octava (Stokes) +V. aequitata (Kahl) apparently followed changes in bacterial activity as indicated by changes in reduction of biochemical oxygen demand and suspended solids of the wastes. This correlation of Vorticella and bacterial activities was also investigated with clonal populations of the vorticellas and their accompanying mixed microflora—the bacteria being isolated together with the vorticellas from sludge samples. The mixed microflora which consistently supported healthy vorticellas was predominantly proteolytic. Varying periods of dominance in Vorticella species thus indicated activity of their supporting microflora. Log‐phase bacteria activity stimulated V. octava; stationary and declining phases stimulated V. convallaria. Thus, V. octava appeared predatory and V. convallaria appeared to require bacterial conditioning of the medium. Bacterial activity was stimulated by V. octava predation. Vorticella activity may thus promote purification.

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