The persistence of alachlor (2-chloro-2′, 6′-diethyl-N-methoxymethylacetanilide) and metolachlor [2-chloro-6′-ethyl-N-(2-methoxy-1-methylethyl)acet-O-toluidide] in soil was determined over an 80-day period at temperatures ranging from 5 to 50°C. Soils were maintained at either 50 or 100% of field capacity. The herbicides were less persistent at high temperatures, but the greater the soil moisture content, the slower the overall degradation rate. The effect of temperature on the phytotoxicity of these two soil-applied herbicides was also determined. The phytotoxic effect of alachlor and metolachlor on beans (Phaseolus vulgarisL. cv. Kamberg) was greatest at 40°C. Beans were not injured at 30°C when treated with metolachlor at 1, 5 I ha−1and alachlor at 4 I ha−1. At higher dosage rates of 3 to 7, 5 I ha−1metolachlor and 8 to 20 I ha−1alachlor, beans showed evidence of injury. At temperatures of 35°C and 40°C, all the dosage rates had adverse effects on the beans.S. Afr. J. Plant Soil1986, 3: 95–98