The preweaning behavioral development was compared in 3 hamster species: the golden (Mesocricetus auratus), Chinese (Cricetulus griseus), and Djungarian (Phodopus sungorus). Differences in the age of appearance of action patterns could not be summarized by simply ranking species on either precocity or developmental speed, nor were they eliminated by measuring age from conception. Problems in the interpretation of species differences in the age of appearance of action patterns are discussed.