The effects of soil depth and moisture on pesticide photolysis were studied. Moist soil at depths of 3, 2.5, 2, 1.5, 1, and 0.5 mm were each dosed at 2.5 microg/g with (14)C-niclosamide and photolyzed under a xenon lamp at constant temperature. Samples were removed after 20, 40, 110, and 153 h of continuous irradiation. The decrease in percent of niclosamide and the appearance of degradates were followed by analyzing the soil extracts by HPLC. A corresponding set of experiments used air-dried soil. An experiment was also performed using initially moist soil which was permitted to dry during photolysis but returned to moist conditions at each sampling. Qualitative and quantitative differences were found in the rate and route of degradation of niclosamide under these conditions. These differences have resulted from a combination of reduced photochemical activity and microbial population in dry soil. The half-lives of niclosamide in the dry soils were 2 to 5 times longer than those in the moisture-maintained soil. There was also a noticeable difference in the half-lives in soil of different depths. Moisture-maintained soil showed a uniform linear increase in half-life from 95 to 195 h as soil depth increased from 0.5 mm to 3.0 mm. With air-dried soil the half-lives were greatly dependent on soil depth, showing a much broader range of 199 h at 0.5-mm to 1064 h in 3.0-mm soil. An experimental design is described which maintains soil temperature and moisture to preset conditions.