Although many animals, including odontocetes, exhibit interactions involving mouths (e.g., mouthing, nuzzling, biting), a limited number of animals display mouth-to-mouth social interactions that involve mutual coordination and collaboration. The purpose of this paper is to describe briefly a spontaneous and unexpected mouth-to-mouth social interaction between beluga calves in human care during their first year of life. Forty-seven independent events were identified after event sampling from more than 345 hr of observations of four mother-calf pairs and their companions. Unique aspects of this behavior included early emergence presumably without the benefit of a model and a preference for similar aged partners. Adult belugas did not display this social interaction. Based on its early emergence, the presence of similar-aged partner preferences, and affiliative cooperative contexts, this unusual mouth-to-mouth social interaction may play a significant role in beluga social and physical development. To evaluate this possibility, additional research is needed in which the complete topography, possible functions, and potential outcomes associated with this rare but conspicuous behavior exhibited by beluga calves are assessed.