SYNOPSIS. Sulfaquinoxaline and ethopabate are 2 chemically distinct types of antagonist of PABA with anticoccidial action. Reversal experiments with PABA and synergism studies with pyrimethamine indicated that both compounds interfered with the PABA-folk acid metabolic sequence. Six pure strains of Eimeriu brunetti responded differently to the 2 compounds. The strain most sensitive to ethopa-WO chemically distinct types of antimetabolite of p-T aminobenzoic acid (PABA) are recognized to have utility in the control of coccidiosis. Analogs of p-aminobenzenesulfonamide (sulfaquinoxaline, for example) represent one type. Nuclear-substituted derivatives of PABA were reported ( 6 , 7 ) to be potent anticoccidial compounds. Ethopabate (rnethyl-4-acetamino-2-ethoxybenzoate) is an example of this 2nd type of PABA antagonist.This paper reports biological comparisons between these 2 related coccidiostats. First, experiments are reported which characterize the 2 compounds as antagonists of PABA. After these initial studies, related to the mechanism of action, the objective was to extend the comparison between the 2 compounds to determine whether a number of isolates of coccidia were correspondingly sensitive to ethopabate and sulfaquinoxaline. A preliminary report of part of the data has been given(5).