1. 1. Extracts of mutant strains of Neurospora crassa which retain only one 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (7-phospho-2-keot-3-deoxy-d-arabino-heptulosonated-erythrose-4-phosphate-lyase (pyruvate-phosphorylating) EC 18.104.22.168) (DAHP synthase) isoenzyme have been used to test the properties of isoenzymes by direct analysis rather than by the differential inhibition analysis previously used for wild-type preparations. DAHP synthases (Phe) and (Tyr), in the presence of their specific negative effectors, show a 40% decrease in molecular weight as estimated by gel filtration. This is in accord with results obtained with wild-type preparations and is interpreted as indicating the dissociation of a polymer into subunits. DAHP synthase from mutants in which the remaining isoenzyme is insensitive to inhibition do not dissociate, implicating dissociation as an essential part of the normal inhibition mechanism. 2. 2. DAHP synthase (Trp) has not been found to dissociate in the presence of tryptophan. 3. 3. Evidence of an in vitro interaction between DAHP synthases (Phe) and (Tyr) has been obtained. The presence of a specific allele of the arom-6 locus (resulting in the loss of DAHP synthase (Tyr) activity) is correlated with a change in the gel-filtration characteristics of DAHP synthase (Phe), the product of an unlinked gene (arom-7).