Eight mutations of the rudimentary-like (r-l) locus were isolated following mutagenesis with ethylmethanesulfonate and inter se crosses revealed three basic complementation groups, using the wing phenotype as an index of complementation. One group consists of three entirely noncomplementing mutants that each specify severe reductions in levels of both r-l-encoded enzymes, orotate phosphoribosyltransferase (OPRTase) and orotidylate decarboxylase (ODCase). The other two groups consist of complementing mutants, such that any member of one group fully complements all members of the other group. One of these groups consist of two mutants that each specify severely reduced OPRTase, but normal ODCase. The other group consists of three mutants that specify severe OPRTase and ODCase reductions in homoallelic flies, but that appear to contribute OPRTase in certain heteroallelic genotypes. It is concluded that the reciprocal and complementing enzymatic phenotypes of mutants in these two groups account for most instances of genetic trans complementation among r-l mutants. These findings are discussed relative to extant information on OPRTase and ODCase in animals and an hypothesis is developed that the r-l locus encodes a single polypeptide product that contains both enzyme activities.