A technique for visualizing particle transport in the interior of a porous medium is presented. The technique, which includes the construction of a translucent medium and the use of laser-induced fluorescence for particle tracking, was used to examine the behavior of a dilute suspension of negatively charged, micron-sized particles in the interior of uniform glass bead packs during one-dimensional, downward flow. Particle behavior as a function of pore fluid velocity and bead surface roughness was observed at both the macroscopic and microscopic levels. Experimental results show that particle filtration occurred only at solid-solid contact points (contact filtration) in smooth bead packs, while particle filtration occurred at the top of bead surfaces (surface filtration) as well as at solid-solid contact points in rough bead packs. Particle contact filtration was the result of physical straining at solid-solid contact points, while surface filtration was the result of particles interlocking on surface asperities. In both smooth and rough bead packs the filtration capacity of the medium decreased with the pore fluid velocity. In smooth bead packs the filtration capacity was approximately invariant with transport distance, while in the rough bead packs the filtration capacity showed a decrease with transport distance. This decrease was attributed to the early surface filtration of larger particles as a result of gravitational sedimentation. The accumulation of reversibly attached particles was observed even when particle pore fluid concentrations were stable.