Isoelectric focusing was investigated to detect esterases of the pest slug Arion lusitanicus in the crop contents of predatory carabid beetles. The method is exceptionally well suited for field studies, as it is fast, cheap, easy to apply, and almost species-specific. The identification of A. lusitanicus was enabled by four characteristic, stable esterase electromorphs. The profile clearly differed from the band patterns of the digestive enzymes of the carabid beetles, from esterases of other potential prey species and even from the enzymes of early developmental stages of the slug. To test the heaviest stained isoenzyme for its decay under controlled conditions, three carabid species were fed on a fixed amount of slug hepatopancreas. In Carabus cancellatus and Pterostichus melanarius the enzyme lost 35\% of its activity after 16 h, fitting a logarithmic curve, whereas in Carabus granulatus the same decay status was already reached after 8 h following a linear regression. Both, the similarities and differences between beetle species might be due to their mode of extra- versus intraintestinal digestion and the amount of hepatopancreas fed. The temperature during digestion was also influential, as the regression slopes of esterase activity between the two temperature regimes tested in C. granulatus were significantly different. The volume of crop contents decreased during digestion in both Carabus species, corresponding to the decline in esterase. In contrast, the crop volume of P. melanarius reached its maximum almost 4 h after ingestion. This was interpreted as an effect of its mode of ingestion and time-elapsed enzyme production. © 2002 Elsevier Science Ltd. All rights reserved.