Fil: Behrends Kraemer, Filipe B. Universidad de Buenos Aires (UBA), Facultad de Agronomía, Cátedra de Manejo y Conservación de Suelos; Argentina.Fil: Chagas, Celio I. Universidad de Buenos Aires (UBA), Facultad de Agronomía, Cátedra de Manejo y Conservación de Suelos; Argentina.Fil: Irurtia, Carlos. Instituto Nacional de Tecnología Agropecuaria (INTA); Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. INIBIOMA-CONICET. Laboratorio Ecotono; Argentina. Universidad de Buenos Aires (UBA). Facultad de Agronomía; Argentina.Fil: Behrends Kraemer, Filipe B. Instituto Nacional de Tecnología Agropecuaria (INTA); Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad de Buenos Aires (UBA). Facultad de Agronomía; Argentina.trueBacterial retention by soils is a key factor in predicting bacterial transport through surface runoff into water bodies. The objective was to evaluate biological, soil and hydrologic factors that affect bacterial retention in three soil types of the Rolling Pampa, Argentina. Simulated rainfall was applied on field plots previously inoculated with Escherichia coli and simultaneously biological variables such as bacterial adsorption and distribution coefficient were measured at laboratory. Soil variables, particularly pH, exchangeable sodium percentage and organic carbon as well as biological variables proved to be important properties in the regulation of bacterial retention processes. There were no significant differences between the biological variables measured in soils and in the sediments. Most of the microorganisms in the runoff from all sites were either free of (in the < 2 μm sediment fraction) or associated with small soil particles (2 to 50 μm), therefore management practices, such as filter strips, should be regarded with caution when implemented.