Pitfall trapping is a sampling technique frequently used by entomologists around the world. However, there exist sampling biases linked to particular trapping designs, which require investigation. In this study, we compared the effects of the type of preservative fluid (propylene glycol or formaldehyde) and the presence of fish bait in pitfall traps on the number of specimens (individuals) collected, the species richness, and the species composition of carabid (Coleoptera: Carabidae) and silphid (Coleoptera: Silphidae) beetle assemblages. Traps containing propylene glycol collected a substantially higher number of individuals of both taxa and a higher number of silphid species compared with traps containing formaldehyde. The use of fish bait in the traps increased the number of individuals collected and the number of species collected for silphid beetles but had no effect on the collection parameters for carabids. The species composition of the carabid assemblages was minimally affected by the presence of fish bait or the type of preservative fluid, whereas the fish bait had a substantial effect on the species composition of silphids. The silphid species that feed directly on vertebrate carcasses were almost completely absent in the nonbaited traps. The results suggest that pitfall traps baited with fish and containing propylene glycol as a preservative fluid are optimal for the simultaneous sampling of carabid and silphid beetles, which both provide important ecosystem services (e.g., predation of pests and decomposition of vertebrate carcasses) and are therefore interesting for ecological research.