Pathogenic bacteria exist at soil surfaces as a result of practices as spreading of liquid manure on agricultural lands or use of treated wastewater for irrigation. Rainfall is a major factor affecting vertical and horizontal movement of bacteria in soil. Surface runoff carries bacteria significant distances downstream causing serious threats to ground and surface waters. This study uses a nalidixic acid-resistant Escherichia coli strain as a biotracer monitoring extent of bacterial migration on sloping soil surfaces by runoff action. Two 10 = 10-m plots in two sites having different slopes were sprayed with water containing biotracer. Soil texture at sites was clay loam. Sixteen days after spraying, two heavy rainfalls that caused runoffs were recorded. First rainfall occurred 2 days after spraying plots. Samples were collected from soil and runoff at different distances downstream of the plots. Biotracer was found in soil and runoff samples some 20 m downstream from center point of plot having the milder slope. Biotracer was found in soil and runoff samples further downstream of the second plot with the steeper slope reaching a 35-and 30-m distance respectively. Most soil and runoff samples collected after the second rainfall, occurring 15 days after inoculation, contained no biotracer except small numbers found in soil samples taken from center point of each plot 5 m downstream. Results confirm the important role of runoff in bacterial transport on soil surfaces. They show E. coli survives in semiarid areas for a long time and increases potential of contamination.