Wasps of the genus Gasteruption are predator-inquilines of bees nesting in cavities in wood, stems, galls, and vertical soil surfaces. During studies of hymenopterans associated with reed galls caused by flies of the genus Lipara we recorded three species. We provide the evidence that a rare European species Gasteruptionphragmiticola is a specialized predator-inquiline of an equally rare wetland bee Hylaeuspectoralis. Gasteruptionnigrescens is a predator-inquiline of bees of the family Megachilidae, using the common bee Hoplitisleucomelana as the main host. Gasteruptionassectator is a less specialized predator-inquiline of multiple groups of bees. The first two species, G.phragmiticola and G.nigrescens, are usually present at well-preserved reed beds associated with meadows rich in flowering plants while G.assectator does not prefer any specific habitat. Mature larvae of Gasteruption typically have a dorsoventrally flattened body and lateral lobes, small head with only slightly sclerotized mouthparts and tridentate mandibles. The larvae of these three species differ only slightly from one another, at most by the sclerotization of the mouthparts, colour, and position of setae on the head capsule and the shape of the mandible. As previous descriptions are insufficient, we provide first detailed descriptions with figures.