We present the results of an outbreak investigation on a Cryptosporidium parvum outbreak among 35 people, (27 students and 8 teachers), who participated in a school excursion to an outdoor adventure farm in South West England, from 22 to 26 May 2006. A cohort study was implemented to investigate possible sources of infection during the farm visit. The most likely transmission route was contact with faecally contaminated surface water following heavy rainfall, or consumption of water from the private well. Disinfection of the water reservoir was by chlorination, to which cryptosporidium is resistant. Supplemental disinfection or filtration methods of private water supplies on livestock farms may be needed. This study highlights the fact that epidemiological investigations of outbreaks as a result of environmental exposures are complex but important to inform the public and health professionals of the risks posed by private water supplies and outdoor activities. This is particularly so after heavy rainfall, as this may result in an increased effluent from faecally contaminated land, causing a wide variety of pathogens to wash into surface water and potentially, private wells. This poses risks for public health.