The locomotory and feeding responses of aEuplotessp. to attached populations ofVibrio natriegensandPseudomonas fluorescensin a continuous flow system were analyzed by computer image analysis of videomicroscopy recordings. Upon entry into the chamber, the ciliates moved in long continuous arcs 300 µm in length during whichtime no bacteria were consumed. As feeding began, the average path length shortened, the arcs became tighter, and the ciliates changed direction more frequently. The feeding activity of theEuplotesappeared to be gregarious, being concentrated in patches within the biofilm of attached bacteria. It was also noted that the feeding effort targeted patches previously visited by otherEuplotes, despite reduced bacterial density relative to the surrounding field of attached bacteria. This focused and intense feeding activity resulted in localized zones of nearly complete clearance within the attached bacterial populations. Loss of bacteria and averaged ciliate presence within feeding patches were determined from digitized time series images and discrimination thresholds for particle size. These data were used to determine grazing rates indicating thatEuplotessp. removed 120V. natriegenscells·ciliate–1·h–1and up to 882P. fluorescenscells·ciliate–1·h–1. However, surface clearance rates forEuplotessp. grazing onV. natriegensandP. fluorescenswere 0.02 and 0.03 mm2·ciliate–1·h–1, respectively, indicating that surface grazing pressure was fairlyconsistent within the patches of intense feeding activity. The effect of such intense localized feeding behaviour on attached orbiofilm bacteria would be to increase spatial and temporal heterogeneity within biofilms.Key words: digital image analysis,Euplotes, grazing, bi