Alachlor persistence was studied in three corn fields from two regions of Spain. The effect of soil temperature and soil moisture content on the rates of alachlor degradation was measured under controlled conditions in soil samples from the same fields. Laboratory data in conjunction with meteorological records were used in two computer models to predict alachlor persistence in the fields. The Walker model was used with a correction on the Linacre equation to take into account the Spanish field conditions. The degradation predicted with this model was faster than the data measured in the fields. The LEACHP v.3 model was used, with a modification in the subroutine that simulates the influence of temperature and moisture content on the degradation component. There was an acceptable agreement between measured and predicted residue levels with the LEACHP model in the three fields. Alachlor persistence and model prediction were discussed.