We investigated parasitism of the purple loosestrife beetle, Neogalerucella calmariensis (L.) (Coleoptera: Chrysomelidae), an introduced weed biological control agent, at 13 sites in eastern Ontario and western Quebec, Canada. Beetles were parasitized by gregarine gut protozoans (Apicomplexa: Eugregarinida) and much less commonly by nematodes (Nematoda: probably Mermithidae) and ectoparasitic mites (Acari: Erythraeidae). Female beetles had more gregarines than did males and the gregarine load was marginally higher in individuals from the original release sites than in individuals from recently colonized sites. Despite its overall success as a biological control agent, N. calmariensis harbours a substantial parasite load.