Soil usually does a good job purifying water from microbial pathogens.  Yet even the relatively large protozoan parasite Cryptosporidium parvum (4 µm diameter) has been found in ground water where manure or waste-water has been applied to the surface.  Transport of C. parvum oocysts decreases when soil water content decreases, possibly due to accumulation of oocysts at air-water interfaces.  Surfactants found in agricultural pesticides and waste-water, as well as natural humic surfactants, decrease the surface tension at the air-water interface and may alter the surface properties of oocysts.   As part of a larger study on the effect of surfactants on transport of Cryptosporidium through soil we evaluated the partitioning of oocysts between soil and surfactant solutions in a batch study.  We determined the rate of oocyst sorption and desorption from four widespread soils (two from Utah and two from Illinois), in four surfactant solutions (cationic HDTMA, non-ionic Tween 80, anionic Aerosol 22, and natural humic acid).  Surfactant concentrations ranged from 0.5 to 2 times the critical micelle concentration.  The results of this study will further our understanding of the mechanisms involved in C. parvum retention in porous media.