The feasibility of modeling the environmental fate and transport of cysts and oocysts of the protozoan pathogensGiardia lambliaandCryptosporidiumspp. in surface waters is examined. A kinetic and modeling framework successfully applied for fecal bacteria is considered for this application. The model utilizes a mass balance approach and accommodates inputs from tributary loads and sediment resuspension, losses to sedimentation and inactivation and mass transport. Recommendations are provided for the design of a program of field monitoring and experimental studies which would support model development. Requirements for model application are considered within the context of Cannonsville Reservoir, a component of the New York City drinking water supply system.