Rapid and accurate sexing of the green June beetle, Cotinis nitida L., is important in research on this species, and secondary sexual characteristics are often used in sexing this insect. However, information about C. nitida secondary sexual dimorphism is limited to a small sample collected from the northeastern United States. We studied the distribution of morphological traits which are thought to be sex-dependent characteristics (number of teeth on anterior tibia, ratio of the hind tibia to the hind tarsus, pubescence of the last abdominal sternite, and general shape of clypeal horn) in a large sample acquired from a Missourian C. nitida population. These beetles exhibited a lesser degree of secondary sexual dimorphism in comparison to northeastern US C. nitida populations, especially in older females that possibly had worn down tibial teeth and abdominal hairs by digging in soil to lay eggs. Consequently, the method of sexing C. nitida by the use of secondary sexual characteristics is probably not applicable for all ages of adult beetles. We illustrate a rapid method of sexing C. nitida that involves a gentle lateral squeeze of the abdomen in vivo to separate the anal tergite and sternite, allowing examination of the primary sexual characteristics (genitalia).