In the field, the female horn fly obtained blood meals from the bovine host before oviposition. The female flies moved to the lower portion of the hind legs as the legs were spread, and the tail was raised before excretion. Females deposited their eggs on the manure after excretion. The fly flew onto the manure pat for oviposition as the cow walked forward after excretion. It appeared that flies located on the belly received some stimuli for oviposition from the cow just before excretion. The horn fly deposited most of its eggs during the day with occasional deposition at night. Egg deposition increased at 10°C. Fewer flies were observed on manure pats that contained >90% or <84% water. The location of horn flies on the cow and environmental factors associated with oviposition in the field are discussed.