SUMMARY: 2,3,5,6-Tetrachloronitrobenzene (TCNB) was a more effective fungistat than pentachloronitrobenzene (PCNB) for Four mutants selected for resistance to TCNB or PCNB were resistant to both these compounds and to other halogenated nitrobenzenes, diphenyl, methylene blue and brilliant cresyl green. Resistance was conferred by either of two recessive genes in linkage group III. Three of the mutants were allelic. In crosses the frequency of resistant ascospores was less than 50%. Diploids heterozygous for resistance exposed to PCNB produced fast-growing resistant haploid or homozygous diploid segregants. PCNB decreased growth more than DNA synthesis in sensitive strains but these were unaffected in resistant strains. Five times as much TCNB was extracted from the mycelium of sensitive strains than from that of resistant strains. Resistance is probably caused by an inability to take up the chemicals rather than to an ability to metabolize them.


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