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Simple Summary: Plant gall-inducing insects can be attacked by a diverse and unique assemblage of predatory and parasitic insects. We surveyed the insect fauna associated with galls induced by the fly Urophora cardui L. on Canada thistle, a perennial weed. We found that the thistle gall fly and its primary parasitoid wasp, Pteromalus elevatus (Walker), were widespread throughout the study area. In addition, we recovered a checkered beetle, Isohydnocera tabida (LeConte), from a subset of galls, which is a new host record and provides vital information on the little-known immatures of this predatory species. This study adds to the taxonomic literature of gall-inhabiting insect species and I. tabida. We surveyed the insect fauna associated with Urophora cardui L. (Diptera: Tephritidae) galls on Canada thistle, Cirsium arvense L. (Asterales, Asteraceae), in parts of the northern Great Plains, U.S., by field-collecting galls and rearing or dissecting out the insects. We also examined the relationships between gall biomass and insect density and biomass. Urophora cardui were widespread, and the gall biomass was positively correlated with fly density and fly biomass. We recovered Isohydnocera tabida (LeConte) (Coleoptera: Cleridae) from galls in two counties, which represents a new host record and provides vital information on the little-known immatures of this predatory species. Pteromalus elevatus (Walker) (Hymenoptera: Pteromalidae) was the dominant parasitoid that emerged from the U. cardui galls. Individual galls typically only had one insect species, and occasionally both U. cardui and P. elevatus were present, but it was rare for other insects to be present in galls housing I. tabida. This study adds to the taxonomic literature of gall-inhabiting insect species and provides new information on the predators of U. cardui, specifically a little-known clerid beetle species. [ABSTRACT FROM AUTHOR]