dog; human-dog interactions; human facial expressions; emotions; ontogeny In 1872, Darwin explored the expressions of emotions in man and animals. The way emotions are displayed is species-specific. Man-dog interactions are unique as they lead to long-term interspecific relationships. During their interspecific interactions humans and dogs should process expressions of emotions of an alien species. Moreover most interactions are multimodal involving simultaneous signals through different compatible channels –visual and vocal. Our study questioned the ability of dogs to understand human still facial expressions in absence of other cues. A sample of 147 medium or small-sized pet dogs (110 adults and 37 non-adults) from various breeds was used in this study. They were exposed to still human facial expressions in an adaptation of a “puppet theater.” Two experimenters, one woman and one man, displayed themselves four basic still facial expressions [anger, joy, disgust, and fear], twice in a random order to each dog, and in absence of other cues (movements and acoustical cues). Dogs were assigned randomly to each experimenter. Each presentation of a facial expression was preceded by a neutral face intended to be used as a “control face”. Each presentation lasted around 5s. in order for the dog to maintain its attention. The behavioral repertoire included 22 behaviors grouped into 3 categories: approach, avoidance and so-called “emotional reaction”. Each dog’s response to a facial expression is compared to those displayed during the “neutral-control face” that preceded it. Therefore, paired data were analyzed. Our results show that both adult and non-adult dogs react more to still facial expressions, especially anger and joy, than to control faces. Non-adult dogs react the same way whatever the human facial expressions that were displayed. All dogs show more avoidance behaviors to the man experimenter whatever the facial expressions he displayed than to the woman experimenter. Only adult dogs react significantly more to anger, in avoiding the human face and to fear in paying more attention to it. Dogs’avoidance reactions to an angry human face thus fit mimicked human emotion. As only adult dogs show these avoidance reactions, this likely suggests that repeated exposure to human expressions of anger is necessary for dogs to learn how to, at least, adjust to a human angry face. The ability in dogs, to react to human facial expressions of emotions is demonstrated with expressive still human faces in absence of other human emotional cues, such as gestural, postural, verbal or vocal. Therefore, this ability, only present in adult dogs, suggests that, during their ontogeny, dogs become able to extract one single component, the facial expression, from the very likely multimodal signaling of an angry human. Our study also show that both adult and non-adult dogs are sensitive to the special configuration of a human face when expressing an emotion. This sensitivity is likely a consequence of the long symbiotic relationships between dogs and humans and of the domestication process.