Technical Abstract: Cryptosporidium parvum is a zoonotic pathogen transmissible from a variety of animals to humans and is a considerable public health concern. Dairy cattle have been identified in numerous reports as a major source of environmental contamination with this pathogen. However, virtually all reports have been based on microscopic examination of the organism in feces from cattle on farms within a limited geographic area. The present study has examined nearly 1000 cattle on 15 farms from 7 states using molecular tools to identify the species and genotypes of Cryptosporidium. Results indicate that virtually all previous reports are misleading in identifying the presence and prevalence of C. parvum by microscopy. Structurally indistinguishable fecal stages to C. parvum were found in calves of all ages in the present study but gene sequencing identified distinct differences. Basically, nearly all of the Cryptosporidium excreted by pre-weaned calves was found to be C. parvum. In contrast, nearly all Cryptosporidium excreted by post-weaned calves were species or genotypes other than C. parvum that have not been found infectious for humans.