Twenty-three exotic strains of four species of Neurospora (N. crassa, N. sitophila, N. intermedia and N. tetrasperma) were grown in minimal medium and their enzyme phenotypes were compared. The exotic strains did not differ substantially with respect to the specific activities of the biosynthetic enzymes, ornithine transcarbamylase (OTC), carbamyl phosphokinase and aspartate transcarbamylase. Exotic strains of N. crassa and N. sitophila were crossed with laboratory mutants of N. crassa to determine whether there were significant differences among the exotic strains in the gene complexes underlying their enzyme phenotypes. Most of the exotic strains carried similar OTC structural genes and similar genetic modifiers of OTC activity: their OTC structural genes were expressed normally in the genomes of the parent strains, in the alien genomes of standard laboratory mutants and in the mixed genomes of parent × mutant hybrids. One exotic N. crassa carried a distinctive OTC structural gene that elevated OTC activity when transferred into the genome of a standard laboratory mutant. A second exotic N. crassa carried distinctive genetic modifiers of OTC activity: two mutations, c- and hi, interacted in the exotic parent to normalize OTC activity and rate of growth, but hi led to extremely high OTC activities and to a reduction in rate of growth when separated from c- by interstrain recombination.