Rove beetles (Coleoptera: Staphylinidae) are important predators of arthropods in soil habitats. However, minimal information is available on their effectiveness, including Dalotia (formerly Atheta) coriaria (Kraatz), which is a reported biological control agent of fungus gnats (Bradysia spp.) in greenhouses. In this study, predation by D. coriaria on Bradysia sp. nr. coprophila, was investigated in small containers (473 ml) in the laboratory using different numbers and ratios of predators and prey. In tests with 1–5 rove beetle adults and 10–40 fungus gnat larvae, predation was greatest at each prey density when four rove beetle adults were released, and lowest at three of four prey densities when five adult rove beetles were released. Per capita prey consumption was greatest when only one rove beetle was present, and predation efficiency decreased as predator numbers increased. This inverse relationship was strongest at the highest prey density (40 fungus gnat larvae). Thus, while using four rove beetle adults in conjunction with 10–40 fungus gnat larvae increased overall effectiveness (number of prey consumed), increasing the number of predators negatively affected predation efficiency. When predator and prey numbers were increased, the level of predation also increased, but only at the highest predator–prey ratio (1:5). At lower predator–prey ratios (1:10 and 1:20), adjusting numbers of predators and prey had no effect on predation. Based on our results, when used appropriately, D. coriaria may be a viable augmentative biological control agent of fungus gnats in greenhouse production systems.