As a consequence of the 1986 Amendments to the Safe Drinking Water Act (SDWA) the U.S. EPA has issued a Surface Water Treatment Rule (SWTR) for systems using surface and ground waters under the direct influence of surface water. In the Guidance Manual to the SWTR, the EPA recommends C x t values (product of disinfection concentration in milligrams per liter and disinfectant contact time in minutes) for different disinfectants to achieve required levels of inactivation for Giardia lamblia. This paper describes the procedure by which C x t values were calculated for Giardia lamblia by chlorine disinfection in the SWTR. A model has been developed which can be used to approximate the C x t values that are embodied in the SWTR. It was found that C x t values increased due to higher pH, the level of inactivation required, and chlorine concentration, and were inversely related to temperature. (Copyright (c) 1993 by Marcel Dekker, Inc.)