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Decimal Elimination Capacity (DEC) of the slow sand filters of the Dutch drinking water Companies was assessed; first by literature review, followed by evaluation of the removal of environmental spores of sulphite-reducing clostridia (SSRC) and small-sized centric diatoms (SSCD) as surrogates. Because these data were not conclusive, additional dosing experiments were performed on laboratory and pilot plant scale. These experiments confirmed the high DEC-values for Cryptosporidium (4.7 log or more) described in literature. The results also showed that the DEC of these filters for persistent organisms can be influenced by long term accumulation and delayed breakthrough. Assessment of the spatial distribution of retained oocysts in the pilot filter showed a significant reduction of oocyst concentration over time. The sand contained a high concentration of zooplankton and on the basis of literature data it was concluded that the oocyst reduction was most likely caused by predation. Predation will be subject for further research. In conclusion slow sand filters have a high DEC for Cryptosporidium and most likely the risk of oocyst accumulation and breakthrough in biologically active and well constructed and well operated slow sand filters is low. Additionally, SSRC and SSCD are no good surrogates to assess the capacity of these filters to remove protozoan oocysts.