Gregarines are parasites common in insects, especially in culture. No effective chemotherapeutic or chemoprophylactic control protocol exists, but sulfadimethoxine has targeted effects on sporozoites and trophozoites. Efficacies of metronidazole and potassium sorbate were independently administered in 5-d regimens to evaluate their efficacy as chemotherapeutic agents against 2 gregarine species, Gregarina cubensis and Protomagalhaensia granulosae, infecting Death's Head Cockroaches, Blaberus discoidalis. Relative to the control, metronidazole significantly reduced intensities of both G. cubensis and P. granulosae but did not significantly reduce gametocyst production. Potassium sorbate had no effect on gregarine intensities but significantly increased the rate of gametocyst production. These results eliminate potassium sorbate as a chemotherapeutic agent in this system but suggest that metronidazole has promise as a gregarine control agent in arthropod monoculture.