When concentrations of bromide ion were determined in waters in a polder district of The Netherlands where there is large-scale use of methyl bromide for soil fumigation, the maximum concentrations in precipitation, surface water and ground water were found to be 0.98, 41 and 17 g/m3, respectively. The highest concentrations of bromide ion in surface water were found in September-October during the main fumigation season. For the period 1 September 1979 to 31 August 1980 a bromide-ion balance was computed for the polder district. The contribution of the use of methyl bromide to the input of bromide ion in the polder district was 215 Mg (Mg = 1000 kg), which corresponds to 68% of the total input of bromide ion (318 Mg). On average over a year, about 14% of the dosage of methyl bromide applied for soil fumigation was converted to bromide ion.