The spatial distribution of Oxytricha bifaria (a hypotrich ciliate) was studied in the laboratory to gain information about a poorly known habitat—the air‐water interface. The results show that this species settles on the interface whenever a bacterial film grows. The ciliate’s locomotory behavior is restricted on newly colonized interfaces but returns to normal patterns on solid substrata when a complete bacterial film develops on the interface. When the bacterial film is broken, ciliate motion is restricted to the micro‐“floes” of bacteria, thus inducing a patchy distribution on the ciliates. Field observations confirm laboratory results. Behavior‐induced patterns in microhabitat distribution may have effects on the reproductive biology of these protozoa.

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The extent to which a particular substrate (scratched glass) affects the distribution of a population (Oxytricha bifaria) that settled on it was measured, yielding the following results: (1) the species distinguishes among substrates slightly differing in the quality of their superficial scratches (average dimensions of 2, 2.8, 3.5, 6.5, 17 and 25 μm, respectively); (2) it “chooses” the preferred substrate by adopting specific differential behaviors; and (3) population cell density conditions its overall distribution.

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The beating cycle of several cirri (frontal cirri 1/0, 3/I, 3/II, 3/III; transverse cirri; one caudal cirrus) of the ciliate Euplotes crassus was studied and described thoroughly in specimens that were actually creeping along the substrate. The beating cycle of the frontal cirri was measured both spatially and temporally, and it was found that (i) the single beating cycle was formed by an active propulsion phase (about 70% of the single step), followed by a recovery phase that so far has never been described and is where the cirri are transferred forwards passively (about 30% of the step); (ii) whenever the euplotes stops, it assumes its "zero position," repositioning all of its frontal cirri to their respective "standard positions"; and (iii) at the beginning of a new creeping phase the frontal cirri were reactivated in a well-defined order. The transverse cirri were kept still during forward creeping, while their angular position was changed with respect to the substrate during the stops and backward movements of the ciliate. The first left caudal cirrus beats constantly and its operating cycle appeared to be independent of the creeping or immobile state of the organism. The findings are discussed from the functional point of view and in the context of available literature on the internal beating potentialities of the different cirri.