Toxicity tests on four annual weeds,Thlaspi arvense L.;Brassica arvensis L. Ktze.;Chenopodium albumL.; andAvena fatuaL.; showed no definite evidence of a specific susceptibility of a given species to a given substance. The relative resistance of these four weeds to most substances, judging from the certainly lethal dose, was in the order 1:1:2:7. Of the 76 chemicals tested, the following most toxic compounds killed all four species at the dosages employed; selenic and chloric acids, sodium hydroxide, arsenic pentoxide, sodium arsenite, sodium and ammonium chlorate, ammonium thiocyanate, sodium cyanide, zinc chloride, sodium bichromate, sodium selenite, copper nitrate, sodium sulphide, formic acid, gasoline, phenol, creosote, tetralin, sodium benzoate, aniline, benzene and furfural. The residual toxic effect on the soil, three to four weeks after treatment, showed that of the 35 more toxic chemicals tested, only selenic acid and the five chlorates used had any appreciable effect at low and intermediate dosages, while eleven other substances depressed growth following the application of high dosages.