Oocysts of C. parvum have been associated with several waterborne outbreaks of gastro-enteric disease. Currently, one of the main barriers to oocyst contamination of drinking waters is provided by sand-bed filtration. In this study an atomic force microscope (AFM) has been used to measure the force of interaction between oocysts of C. parvum and a model sand surface (silicate glass). The AFM force curves have been compared and contrasted with the corresponding electrical potentials obtained from electrophoretic measurements (zeta). It has been found that the surface of C. parvum oocysts possesses a hairy layer, most likely a result of surface proteins extending into solution. The hairy layer imposes a steric repulsion between the oocyst and sand surface, in addition to any electrostatic repulsion. The hairy layer collapsed to varying extents in the presence of dissolved calcium and dissolved organic carbon, indicating that the oocysts may be more readily adsorbed onto the model sand surface under these conditions. Conversely, as the two surfaces are pulled apart, the occasional attachment of oocyst surface proteins to the model sand surface can result in adhesion. The AFM results offer new insights into the oocyst surface of C. parvum, and the mechanism of interaction with model sand surfaces under conditions relevant to sand-bed filtration.