Studies of dog personality have shown that personality concepts can be applied to dogs but suggest that canine personalities may not fall into the same dimensions as do human personalities. To investigate this, the structure of canine personality was explored using a method previously used to characterize human personality. A large number of adjectives believed by experts and companion dog owners to potentially describe canine personality traits were examined by the members of two focus groups, who identified 203 adjectives thought to be applicable to companion canines. These adjectives were rated by 92 participants in a pilot study and the number of words reduced to 67 using statistical and theoretical principles. Over 1000 owners then rated their companion dog on these 67 personality adjectives. Principal component analysis revealed five underlying factors that accounted for 32.6 of the total variance. Two of these, extraversion (8.3 of variance) and neuroticism (4.6 ), are similar, but not identical, to dimensions identified in other species. The remaining three, tentatively labeled self-assuredness/motivation (6.5 ), training focus (6.7 ) and amicability (6.4 ), may be unique to canines and reflective of the strong and unique selective pressures exerted on this species by humans.