This study compared three methods used to measure aggression in the male three-spined stickleback by using them to measure changes in behaviour over the twelve days following the fertilization of eggs. These methods have been used in various studies without the certainty that they measure the same behavioural phenomena. In addition a number of different, possible measures of aggression were taken in the hope that this would show whether a unitary drive concept of aggression was realistic. The methods consisted of recording the behaviour of experimental males in one of three situations: (i) a tube containing another male was visible for five minutes, (ii) another male was visible through a glass partition at all times, (iii) a fish-shaped wax model was visible for two minutes. Each method showed that in the twelve days after the fertilization of eggs, there was a U-shaped trend in the frequency of biting, frequency of charging and in the rate of biting per minute of time spent oriented towards the test stimulus. In Method (i), the total oriented time took up about 70% of a test period, was not correlated with frequency of biting and did not show a U-shaped trend. In Method (ii) and (iii) the total oriented time formed an average less than 50% of a test period, was correlated with frequency of biting and did follow a U-shaped trend over the 12 days. In both Method (iii) and especially Method (ii) fanning was an important component of the test period, notably in the middle of the 12 day period. There were no comparable trends in either spine-raising or threatening. The agreement between Methods (i) and (ii) was very close for the frequency and rate measures but not for the duration measures. Only one model, that painted red ventrally and silver dorsally, showed a significant degree of agreement with Methods (i) and (ii). The measure rate of biting per minute of oriented time, which is independent of both the length of the test period and the total oriented time, showed a very high level of agreement in comparisons between the three methods. Allowing for the differences inherent in the methods it was concluded that there probably was a common causal factor for the changes in aggression seen over the parental period and measured in the three different test methods. The literature and personal observations suggest that these changes are paralleled by changes in territorial size over the parental period. Behaviour such as spine-raising and threatening did not show similar changes and so may not be affected by the same factor or factors.