SYNOPSIS.Nosema parkeri sp. n. is described from nymphs and adults of the argasid tick,Ornithodoros parkeriCooley, from a laboratory colony. Schizogonic and sporogonic stages are described from various tick tissues. Spores are binucleate, measuring 3.2 (3–4) × 1.9 (1.8–2.5) μm. Transmission is transovarial and transstadial. The parasite does not appear to affect adversely the development or reproduction of the tick.Dermacentor andersoniStiles was experimentally infected. Attempts to infect Swiss mice by tick feeding or by injection of infected tick suspensions were unsuccessful. The microsporidan differs in structure fromEncephalitozoon ixodisWeiser) andNosema slovacaWeiser&Reháček, the only other microsporidans known fro