Heterokaryons ofNeurospora crassa were generated by transformation of multinucleate conidia of ahistidine-3 auxotroph withhis-3+ plasmid. In one of the transformants, propagated on a medium with histidine supplementation, a gradual but drastic reduction occurred in the proportion of prototrophic nuclei that contained an ectopically integratedhis-3+ allele. This response was specific to histidine. The reduction in prototrophic nuclei was confirmed by several criteria: inoculum size test, hyphal tip analysis, genomic Southern analysis, and by visual change in colour of the transformant incorporating genetic colour markers. Construction and analyses of three-component heterokaryons revealed that the change in nuclear ratio resulted from interaction of auxotrophic nucleus with prototrophic nucleus that contained an ectopically integratedhis-3+ gene, but not with prototrophic nucleus that containedhis-3+ gene at the normal chromosomal location. The growth rate of heterokaryons and the activity of histidinol dehydrogenase—the protein encoded by thehis-3+ gene-remained unchanged despite prototrophic nuclei becoming very scarce. The results suggest that not all nuclei in the coenocytic fungal mycelium may be active simultaneously, the rare active nuclei being sufficient to confer the wild-type phenotype.