Fruit-colour polymorphisms are common in nature, but their genetic bases have rarely been examined in wild species. Here, I report on controlled crosses in Acacia ligulata A.Cunn. ex Benth., an Australian arid-zone shrub with a red–yellow–orange aril colour polymorphism. The evidence is consistent with 1-locus, 2-allele control of red v. yellow phenotypes; these phenotypes comprise 98.7% of the adult plants in nature. At this proposed r locus, yellow is dominant to red. Evidence concerning the rare orange morph is limited, but is consistent with models in which orange is produced by either (a) a third allele at the r locus or (b) modification by a second locus. Simple genetic architecture for ecologically relevant traits, such as fruit colour, should aid in linking ecological processes such as frugivory and seed dispersal to the evolutionary trajectories of plant populations.