Abstract.Ommatidium lens structure in the Trogidae and the monotypic Glaresidae is studied. Three types (eucone, duocone and exocone) are found in the Trogidae [and most Scarabaeoidea]; only the eucone type occurs in the Glaresidae. Evidence is presented that evolution of ommatidium type in the Trogidae has proceeded from a basic eucone type such as that in extantGlaresisandTrox, to a more derived eucone as inPolynoncusand some species of theOmorgussubgeneraOmorgusandAfromorgus, through various states of the duocone type as in the remainder of the species belonging to these subgenera, to the highly evolved exocone ommatidium found in the monobasicOmorgus (Haroldomorgus).An improvement in optics of the duocone and exocone over the eucone may have driven the evolution of these types. Ommatidium structure does little to confirm or refute proposed relationships among trogid taxa but does clearly indicate the path of evolution of the different lens systems in the Trogidae and illustrates the probable course of evolution of ommatidium structure in the Scarabaeoidea.