The expertise on the interpretation of and 11 b TierSchG assumes that a hypertrophy of aggressive behaviour exists in some blood lines of Bull Terriers, American Staffordshire Terriers and Pitbull type dogs. This study was carried out to detect whether a hypertrophy of aggressive behaviour occurred in a certain Bull Terrier breed line. A total of 38 dogs representing this line were tested according to the guidelines of the Dangerous Animals Act of Lower Saxony, Germany (GefTVO) enacted on July 5th, 2000. Furthermore, the results of their behaviour towards other dogs during the test were compared to those of 347 dogs tested by Böttjer (2003) in order to investigate possible significant differences in the occurrence of inadequate or disturbed aggressive behaviour. The comparison was aimed at exposing a possible significant accumulation of intraspecific aggressive behaviour. In the situations of dog-dog-contact of the test, 25 threatening behaviour"was displayed by 9 dogs (23.68%). Four dogs (10.53%) responded with "non-stationary threatening behaviour". All Bull Terriers reacted appropriately in every situation. A significant difference when comparing the results of the Bull Terriers to those of the dogs examined by Böttjer (2003) was not found. In conclusion, there were no indications for inadequate or disturbed aggressive behaviour in this Bull Terrier breed line. Furthermore, the broad majority of dogs proved to possess excellent social skills as well as the ability to communicate competently and to solve conflicts appropriately.