Ninety-five percent of the larvae of Contracaecum spp. were found in the atrium of the heart of fathead minnows (Pimephales promelas) and five- and nine-spined sticklebacks (Culaea inconstans and Pungitius pungitius). Atria containing nematodes were two to three times larger than an atria from uninfected fish, were thin walled and the muscular trabecule were damaged. Atria with larvae were congested with blood that appeared to be organizing and extending as a thrombus into the ventricle, but there was no other evidence of a host response. Alternatively, there was an intense cellular response surrounding larvae in the body cavity of these and other host species. Apparently, this parasite which lacks host specificity has evolved an additional strategy to evade the host immune response in sticklebacks and fathead minnows.