ABSTRACT Culture filtrate from Pyrenophora tritici-repentis race 1 isolate 78-62 contained a genotype-specific toxin which elicited extensive chlorosis on sensitive wheat genotypes. This toxin was partially purified using gel filtration, ion exchange, and reversed-phase chromatography. The chlorosis toxin was found to be a polar, nonionic, low-molecular-weight molecule. Wheat genotypes infiltrated with crude culture filtrate and partially purified chlorosis toxin exhibited the same chlorotic symptoms seen with conidial inoculations of isolate 78-62. All tested wheat genotypes that exhibited extensive chlorosis to the toxin also exhibited extensive chlorosis to conidial inoculations, and all wheat genotypes insensitive to the toxin did not exhibit extensive chlorosis to conidial inoculations. The recombinant inbred population derived from the cross W-7984 x Opata 85 segregated for chlorosis induction from infiltration with partially purified chlorosis toxin from isolate 78-62. The locus identified by the marker XGli1, associated with resistance to conidial inoculations from race 1 isolates Pti2 and 78-62 and race 3 isolate D308, also was associated with insensitivity to infiltration of crude culture filtrate and partially purified chlorosis toxin. The marker XGli1, located on the short arm of chromosome 1A, is linked to the insensitivity locus within 5.7 cM. We propose that this chlorosis toxin be designated Ptr ToxC.