The marine ciliate Euplotes crassus was exposed to different sublethal concentrations of mercury, cadmium and chromium, and its behaviour was video-recorded and analysed. It was found that the rhythmic changes in creeping velocity, already known as Slow Down, were affected by each heavy metal, inducing an increase in their duration and frequency. In particular, the ratio between the time spent by Euplotes performing the Slow Down patterns and the duration of the remaining part of the track (Slow Down Index) accurately expresses the effects of the different toxicants. The Pondered Effect Index, which is calculated on the basis of the single Slow Down Index values, to compare control and treated experimental populations, is proposed as a tool that reliably measures the effects of the toxicants on these ciliates.