In this paper, we postulate that uncertainties associated with nonpoint source pollution and with certain features of pollutant trading may tend to outweigh the potential benefits of pollutant trading from the standpoint of regulatory agencies and point source dischargers. After identifying several major sources of such uncertainties, we outline a trading scheme for a typical phosphorus-polluted water body in an agricultural watershed. The scheme limits uncertainty while preserving the efficiency-seeking features of trading to effect water quality improvements.