The aim of the present study was to evaluate the heritability of defence capacity traits in the Belgian shepherd dog and to calculate the genetic correlations between traits and the role of particular environmental factors. The study used 15 772 competition results from defence dogs involving 2427 Belgian shepherd dogs in France from 1986 to 1996. A competition included 6-19 different tests and according to their difficulty five levels. The tests were grouped together in order to class the dogs in eight general ability measures: Jumping, Following at heel, Fetching an object, Attacking, Guarding, Obedience, Biting and Global success. The analysis was performed on the calculated scores after the dogs had been ranked within a competition. The scores were corrected according to the average level of the dogs participating in the competition. This method is used in horses, the "performance rate". This was used to produce scores, which had a normal distribution. The genetic parameters were estimated using a mixed animal model using the Restricted Maximum Likelihood method (REML). The fixed effects of the model were estimated by the Best Linear Unbiased Estimation (BLUE) and their significance by an F-test. The heritability estimates are low for Following at heel (h2=0.07) and Global success (h2=0.07) but moderate for the other criteria (h2=0.13-0.18). The repeatability of results was relatively high (r=0.39-0.59). The phenotypic correlations between abilities were low to moderate, however, the genetic correlations were moderate to high, except for Jumping which appears to be independent from the other abilities. The males performed better than the females. The Malinois was the best variety of dog. The effect of age was studied within each level of competition difficulty. The best results were obtained as early as 1.5 years-of-age for level 1 and between 3 and 7 years-of-age for levels 4 and 5.