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Notes: Summary Wild type and 22 mutant strains of Neurospora crassa were compared for L-glutamine: D-fructose 6-phosphate amidotransferase (GFAT: EC 2.6.1.16) specific activity. The mutant genes map at eleven functionally nonallelic loci, mutations at which result in altered mycelial and ascus morphologies. The enzyme GFAT, which is involved in the synthesis of the important mycelial wall constituent chitin, was investigated as a possible causal factor in determining the altered morphologies. When compared with the wild-type (control) activity, a statistically increased GFAT activity was detected in crude extracts of six mutant strains. Two of these strains carry mutant alleles mapping at the peak locus (peak-2 and clock) and the remaining four carry mutant genes mapping at four other loci. GFAT activity was not significantly different from that of wild type in crude extracts prepared from nine other strains carrying mutant alleles that map at the peak locus, or from seven strains carrying the remaining mutant alleles mapping at other loci. In the two cases tested, including that of peak-2, the activity increase segregated with the mutant morphological phenotype when cultures derived from ascospores of tetrads isolated from crosses of the mutants with wild type were assayed. Further studies with crude extracts showed that, compared with wild type, GFAT activity of peak-2 is higher at all growth times between 12 and 72 hours, and the enzyme is more thermolabile at 35°C. The higher GFAT activity of peak-2 was shown probably not to be due to differential feedback inhibition. The peak locus is almost certainly not a structural gene for the enzyme GFAT, since only two out of the eleven strains carrying mutant alleles at the peak locus that were assayed for GFAT activity showed increased activity compared with that of wild type. For the four nonallelic mutants showing higher GFAT activities than wild type, evidence is still insufficient to determine whether or not the appropriate loci are structural genes for GFAT. The increased GFAT activity where found is consistent with, and possibly contributory to, the altered morphologies.