This study examines the effects of sex and familiarity on Americans’ talk to dogs during play, using categories derived from research comparing mothers’ and fathers’ talk to infants. Eight men and fifteen women were videotaped whilst playing with their own dog and with another person’s dog, and their utterances were codified for features common to infant-directed talk. Women used the baby talk speech register more than men, and both men and women used this register more when interacting with the unfamiliar dog than with the familiar dog. When playing with the familiar dog, women talked more than men, and their talk was more suggestive of friendliness and having a conversation. When playing with the unfamiliar dog people used more praise, more conversational gambits, a more diverse vocabulary, and longer utterances than when playing with the familiar dog, suggesting that when playing with the unfamiliar dog, people pretended to have more of a conversation, were more attentive to appearing friendly and were less attentive to the dog’s limited understanding. Overall, however, men and women used similar forms of talk when interacting with a dog, whether familiar or not.