Thirty-nine saturated fatty acid requiring mutants were isolated after treatment with ethyl methanesulfonate (EMS). These mutants have been assigned to three major complementation classes and from tetrad analysis appear to represent three unlinked genes: at least two loci are not linked to any centromere. Two major complementation categories are delineated by two non-allelic mutnats. Though fully complementary to each other, each fails to complement any mutant in several different complementation subgroups. Characterized by their response to external suppressors, these mutants represent nonsense mutations. One mutant is of the amber variety and the other is ochre. Tetrad analysis of diverse hybrids involving the saturated fatty acid auxotrophs revealed cryptic aneuploidy for chromosomes III, VIII and the as yet unidentified chromosomes that carry the above ochre and amber mutants. Clearly, this represents a minimal estimate. A more precise evaluation requires inclusion of sufficient genetic markers adequate to monitor the entire yeast genome. Gene dosage alterations due to aneuploidy had no apparent effect on the efficiency of interallelic complementation. The implications of these findings on the regulation and assembly of fatty acid synthetase are discussed. A working hypothesis concerning the relation between the effect of the primary lesion and nondisjunction is presented.