Artificial cow pats were seeded with Cryptosporidium oocysts and subjected to a simulatedrainfall event. The runoff from the faecal pat was collected and different particle size fractionswere collected within settling columns by exploiting the size-dependent settling velocities.Particle size and Cryptosporidium concentration distribution at 10cm below the surface wasmeasured at regular intervals over 24 h. Initially a large proportion of the total volume of particlesbelonged to the larger size classes (.17mm). However, throughout the course of the experiment,there was a sequential loss of the larger size classes from the sampling depth and apredominance of smaller particles (,17mm). The Cryptosporidium concentration at 10cm depthdid not change throughout the experiment.In the second experiment samples were taken from different depths within the settling column.Initially 26% of particles were in the size range 124–492mm. However, as these large particlessettled there was an enrichment at 30cm after one hour (36.5–49.3%). There was a concomitantenrichment of smaller particles near the surface after 1 h and 24 h. For Pat 1 there was no differencein Cryptosporidium concentration with depth after 1 h and 24 h. In Pat 2 there was a difference inconcentration between the surface and 30cm after 24 h. However, this could be explained by thesettling velocity of a single oocyst. The results suggested that oocysts are not associated with largeparticles, but exist in faecal runoff as single oocysts and hence have a low (0.1md21) settlingvelocity. The implications of this low settling velocity on Cryptosporidium risk reduction withinwater supply reservoirs was investigated through the application of a three-dimensional model ofoocyst fate and transport to a moderately sized reservoir (26 GL). The model indicated that the roleof settling on oocyst concentration reduction within the water column is between one and threeorders of magnitude less than that caused by advection and dilution, depending on the strength ofhydrodynamic forcing.