The bee genus Pseudoanthidium is represented by nine species in Europe. Of these nine species, Pseudoanthidium nanum is the most widespread, occurs mainly in xerothermic open habitats and creates nests in various cavity types. In this study, we provide information on the nest structure of this species in reed stalks and oak galls and about its parasitic species. We provide the first report of P. nanum as a host of Xylophrurus augustus (Ichneumonidae). The biology of the much rarer related species Pseudoanthidium tenellum is described here for the first time. This species occurs in terrestrial reed beds and wet meadows with the presence of reed galls and flowering plants in the family Asteraceae and is rare throughout its entire distribution area. This species nests inside reed galls induced by Lipara frit flies, and the nest structure is very similar to that of P. nanum. We report new parasitic species of this bee, namely, the cuckoo bee Stelis punctulatissima, the predator-inquiline Gasteruption nigrescens and two parasitoids, Leucospis biguetina and Miltogramma punctata. This bee collects pollen mainly from wetland plants in Bidens and Pulicaria. We also describe mature larvae of both species. The larvae do not differ greatly from one another; only the shape of mandibles and sclerotisation of mouthparts are slightly different. Further research should address the ecological requirements of P. tenellum, a poorly understood reed gall inquiline.