One-hundred-and-twelve small animal veterinarians and 56 dog care professionals were asked to rate the behavioural characteristics of 49 breeds of dog, and to compare males and females by means of a 13-point questionnaire. From their replies, factor analysis was used to extract three underlying traits, labelled aggressivity, reactivity and immaturity. On the basis of these traits, eight groups of breeds were derived. Membership of these groups did not correspond exactly with any of the four existing breed classification systems (Mégnin, the Fédération Cynologique International, ancient breeds and Kennel Club of Great Britain), but significant differences between Kennel Club groups were found on all three traits. Male dogs were rated higher than females on both aggressivity and immaturity; the components of reactivity were also rated higher in males, apart from the demand for affection which was rated higher in females. Females were also considered easier to train than males.