Infections resulting from administration of a single oocyst isolated from an amprolium-resistant strain of Eimeria tenella were established in 5 of 32 chicks. Oocysts recovered from these infections were propagated in unmedicated birds and the progeny tested for resistance to amprolium. All isolants were resistant to the chemical. The Anticoccidial Index calculated for infections initiated with 1 isolant was lower in each of the 3 trials. Most anticoccidial agents suppress development of the parasites rather than destroy them. Withdrawal of many coccidiostats allows some of the parasites to recover and to complete their life cycle provided that, in general, (1) the initial exposure is great enough and (2) the period of medication is relatively short (Reid, Taylor, and Johnson, 1969; Reid et al., 1970; Ryley and Wilson, 1971; and others). Moreover, for economic, pharmacologic, and immunologic reasons, coccidiostats are fed continuously at the lowest level that will prevent expression of frank disease. At these levels, at least a small part of the parasite population usually will develop to patency even though clinical disease is not evident. Coccidia respond to continuous low-level medication, as do many other microorganisms, by developing a tolerance for or resistance to the drug in question. Once developed, resistance persists virtually unchanged when the strain is passed serially through groups of unmedicated birds (McLoughlin and Gardiner, 1963), although repeated exposure to another chemical may, in some instances, alter the resistance pattern (McLoughlin, 1971). Once a strain has developed a stable resistance to a given compound, presumably all members of the population are resistant because those that failed to develop a tolerance would not have survived repeated drug exposure. If this presumption is true, then are all members of the population equally resistant? Using an amprolium-resistant strain of Eimeria tenella as a model, we began the Received for publication 4 March 1974. * Animal Parasitology Institute, ARS, Beltsville, Maryland 20705. present studies to determine if, indeed, all members of the population are resistant and, if so, to determine if they are equally re-