Overland and shallow subsurface hydrologic transport of pathogenic Cryptosporidium parvum oocysts from cattle feces into surface drinking water supplies is a major concern on annual grasslands in California's central and southern Sierra Nevada foothills. Soil boxes (0.5 m wide x 1.1 m long x 0.3 m deep) were used to evaluate the ability of grass vegetated buffer strips to retain 2 x 10(8) spiked C. parvum oocysts in 200-g fecal deposits during simulated rainfall intensities of 30 to 47.5 mm/h over 2 h. Buffers were comprised of Ahwahnee sandy loam (coarse-loamy, mixed, active, thermic Mollic Haploxeralfs; 78:18:4 sand to silt to clay ratio; dry bulk density = 1.4 g/cm(3)) set at 5 to 20% land slope, and >/=95% grass cover (grass stubble height = 10 cm; biomass = 900 kg/ha dry weight). Total number of oocysts discharged from each soil box (combined overland and subsurface flow) during the 120-min simulation ranged from 1.5 x 10(6) to 23.9 x 10(6) oocysts. Observed overall mean log(10) reduction of total C. parvum flux per meter of vegetated buffer was 1.44, 1.19, and 1.18 for buffers at 5, 12, and 20% land slope, respectively. Rainfall application rate (mm/h) was strongly associated with oocyst flux from these vegetated buffers, resulting in a decrease of 2 to 4% in the log(10) reduction per meter buffer for every additional mm/h applied to the soil box. These results support the use of strategically placed vegetated buffers as one of several management strategies that can reduce the risk of waterborne C. parvum attributable to extensive cattle grazing on annual grassland watersheds.