Methylation of applied divalent mercury ion was found to occur in agricultural soils. The production of methylmercury was affected by soil texture, soil moisture content, soil temperature, concentration of the ionic mercury amendment, and time. Methylation was directly proportional to clay content, moisture content, temperature, and mercury concentration. After an initial build-up of methylmercury in soil, there appeared to be a mechanism that decreased the methylmercury content with increasing time. Copyright (c) 1976, ASA, CSSA, SSSA.