SummaryField scale experiments carried out over three years showed that chlorpyrifos reduced the numbers of several carabid beetle species caught in pitfall traps. Fonofos seed treatment was also shown to affect selected species. Consideration of the biology of individuals of groups of species provided explanations of why some species such asNebria brevicollis, Notiophilusspp.,Bembidionspp., andTrechus quadristriatuswere severely affected by one or other of the chemicals, whereas other species (e.g.Amaraspp.,Harpalusspp.) were able to survive or recolonise pastures shortly after chemical application. Measurements of elytral and pronotal widths and counts of mature eggs inN. brevicollis, which was trapped in reduced numbers over a long period, were made and provided some information about the mode of action of these chemicals.