The conversion of aldrin to the more residual and equally toxic material, dieldrin, has been found to occur On alfalfa, soybeans, and corn and may take place on plants in general. A 1-pound per acre treatment of aldrin on alfalfa yielded aldrin residues which had dissipated to 0.1 p.p.m. in 8 days after spraying while the dieldrin produced took 25 days to reach the same level. This conversion has been proved by specific colorimetric, total organic chloride, bioassay, paper chromatographic, and infrared spectroscopic methods.