Some pelagic flagellates colonize particles, such as marine snow, where they graze on bacteria and thus impact the dynamics of the attached microbial communities. Particle colonization is governed by motility. Swimming patterns of 2 particle-associated flagellates, Bodo designis and Spumella sp., are very different, the former swimming slowly in an erratic, random pattern, and the latter faster and along smooth helixes of variable amplitude and frequency. At spatial scales exceeding ca. 50 mum, the motility of B. designis can be described as a random walk and modeled as diffusion. Spumella sp. shows directional persistence of the helical axes up to a scale of at least about 0.5 mm, and its motility cannot, thus, be characterized as a random walk at such small scales. Motility analyses predicted overall rates at which the 2 flagellates encountered and colonized model particles (4 mm agar spheres) rather well. After initial colonization, the number of flagellates remained approximately constant for similar to 10 h or more. In B. designis this was due to a density-dependent attachment probability, while in Spumella sp. the declining accumulation rate was better explained by a constant specific detachment rate and a constant (low) attachment probability (12%). The grazing impact of B. designis on attached bacteria was estimated from short-term (4 to 8 h) differences in development of attached bacterial populations in the presence and absence of the flagellate. B. designis ingested up to 120 bacteria ind.(-1) h(-1) (ingestion rate increasing with increasing density of bacteria on the sphere) and had surface area clearance rates of up to 1.3 x 10(-4) cm(2) h(-1). At flagellate densities typical of marine snow, the implied bacterial grazing mortality exceeds bacterial growth and colonization, suggesting that flagellate grazing controls abundances of attached bacteria.