A quantitative description of copulatory behavior in Calomys callosus (laucha de campo) was obtained under laboratory conditions. The copulatory pattern is characterized by intravaginal thrusting and multiple intromissions preceding ejaculation. Copulatory locks were rare. Copulatory behavior typically ceased upon attainment of the first ejaculation. Their small number of ejaculations per test resembles some other South American cricetines.