The BioSand filter (BSF) is a point-of-use treatment technology used in developing countries to improve drinking water quality. Studies on the BSF to date have mainly focused on bacterial, protozoan and virus reduction and less on the filter's effect on chemical contaminants that may also affect human health. This study explored the potential for nitrification in field-scale trials of four BSFs treating water from a covered drilled well in rural Cambodia. The effect of raw water ammonia concentration (0.4 to 18 mg-N/L) and filter idle time (2 and 24 h) were investigated. Incomplete nitrification was observed in both control and test BSFs as inferred from nitrite and nitrate measurements. At the highest ammonia loading (18 mg-N/L), the sum of ratios of nitrite and nitrate in BSF effluent exceeded the World Health Organization's limit by as much as 40%, but only due to the presence of 1.5 mg-N/L nitrate in the influent. Three methods for minimizing nitrite and nitrate in filtered water were evaluated, including wasting certain fractions of filtered water that may have higher concentrations, cleaning the BSFs and allowing filtered water to aerate for a period of time. None of these methods were deemed to sufficiently counteract potential nitrification.