The host specificity of Gregarina blattarum was evaluated among five species of domiciliary cockroaches: Blatella germanica, Supella longipalpa, Blatta orientalis, Periplaneta americana, and Periplaneta fuliginosa. Third- and fourth-instar nymphs were allowed to feed on crushed dog kibble contaminated with G. blattarum oocysts. Cockroaches were killed 8 days postinfection and examined for parasites. Gregarina blattarum infections were observed in all experimentally infected replications of B. germanica. No infection was observed in an experimentally infected replication of S. longipalpa, B. orientalis, P. americana, or P. fuliginosa, nor was an infection observed in a time zero or ending time control group. In vitro excystation assays using extracts of host gut homogenates demonstrate that G. blattarum sporozoites successfully excyst and begin the life cycle in all five cockroach species tested. No excystation was observed in neutral buffered saline controls. These data suggest that G. blattarum comprises a complex of cryptic species marked by narrow host utilization rather than a single species parasitizing a broad array of cockroach taxa.