The aim of the study was to determine whether there is an association between the personality of the owners of English cocker spaniels and the expression of aggressive behaviour by their dogs. Two-hundred-and-eighty-five owners of purebred English cocker spaniels completed the Catell 16 Personality Questionnaire. One-hundred-and-twenty-eight of them owned 153 dogs previously classified as being 'low' in terms of aggressiveness and 157 owned 172 dogs classified as being 'high' in terms of aggressiveness. Both groups of owners were similar in terms of a variety of demographic variables, including the number of adults and children in the household, the type of house and the sex of the owner. The dogs in both groups were similar in age, age when acquired and sex ratio. Analyses of the data using unpaired t-tests revealed that the owners of high aggression dogs were significantly more likely to be tense (P < 0.001), emotionally less stable (P < 0.01), shy (P < 0.01) and undisciplined (P < 0.05) than owners of low aggression dogs.