ABSTRACTOur primary goal was to generate an accurate estimate of the daily environmental loading rate ofCryptosporidium parvumoocysts for adult beef cattle, using immunomagnetic separation coupled with direct immunofluorescence microscopy for a highly sensitive diagnostic assay. An additional goal was to measure the prevalence and intensity of fecal shedding ofC.parvumoocysts in pre- and postparturient cows as an indicator of their potential to infect young calves. This diagnostic method could detect with a ≥90% probability oocyst concentrations as low as 3.2 oocysts g of feces−1, with a 54% probability of detecting just one oocyst g of feces−1. Using this diagnostic method, the overall apparent prevalence of adult beef cattle testing positive forC.parvumwas 7.1% (17 of 240), with 8.3 and 5.8% of cattle shedding oocysts during the pre- and postcalving periods, respectively. The mean intensity of oocyst shedding for test-positive cattle was 3.38 oocysts g of feces−1. The estimated environmental loading rate ofC.parvumranged from 3,900 to 9,200 oocysts cow−1day−1, which is substantially less than a previous estimate of 1.7 × 105oocysts cow−1day−1(range of 7.7 × 104to 2.3 × 105oocysts cow−1day−1) (B. Hoar, E. R. Atwill, and T. B. Farver, Quant. Microbiol.2:21-36, 2000). Use of this highly sensitive assay functioned to detect a greater proportion of low-intensity shedders in our population of cattle, which reduced the estimated mean intensity of shedding and thereby reduced the associated environmental loading rate compared to those of previous studies.