Removals of bacteria and Giardia cysts in a pilot plant sedimentation basin were evaluated and compared with turbidity removal. Removal was studied during normal operating conditions, during initial period of filter operation after backwashing, and during times of turbidity breakthrough. It was concluded that continuous flow sedimentation basins can remove a substantial fraction (perhaps up to 90 percent) of bacteria or cysts in the applied water. Microorganism reduction may be similar to turbidity reduction on a percentage basis. The tendency of filters to produce water with somewhat elevated turbidity upon restart after backwashing is accompanied by a tendency to pass higher concentrations of cysts and bacteria at the same time. Pilot plant data showed that the probability of attaining high numbers of bacteria or Giardia cysts in filtered water was greater when filtered water turbidities ranged from 0.21 to 1.4 ntu in contrast with data with filtered water turbidities not exceeding 0.20 ntu.