This study highlights the value of museum collections in invasion biology. It focuses on introduced tilapias, Oreochromis niloticus and Coptodon rendalli in the Congo Basin and their monogenean (Platyhelminthes) gill parasite fauna. O. niloticus was introduced throughout the Congo Basin while C. rendalli was introduced into the Lower Congo, but is native to the Middle and Upper Congo. In order to study the impact of these stocking events on the native parasite community we investigate the co-introduction and host switching of their parasites. Post-introduction material is compared with pre-introduction samples from museum collections of 5 native tilapias in the Congo Basin. Nine of the known parasites of O. niloticus were co-introduced, while one, Cichlidogyrus rognoni, is missing and possibly not established. In contrast, no parasite species were found co-introduced with C. rendalli into the Lower Congo. The parasite fauna of Tilapia sparrmanii shared no species with O. niloticus. Oreochromis mweruensis shared five species with O. niloticus, but these were also found on the pre-introduction samples, and are considered native to both hosts. We report three putative host switches: Cichlidogyrus sclerosus and Cichlidogyrus tilapiae to Coptodon tholloni in the Lower Congo Basin and Gyrodactylus nyanzae to C. rendalli in the Upper Congo