Deep bed filtration has been studied experimentally and numerically for small non‐Brownian particles flowing into a random packing of monosize glass spheres at low Reynolds number. It was discovered that packets of particles penetrated further than the same number of particles released one at a time. These collective effects are attributed to hydrodynamic phenomena, one plausible explanation being the existence of relaunchable ‘‘hydrodynamic captures’’ in addition to ‘‘geometric captures.’’ ©1996 American Institute of Physics.