1. It is demonstrated that the Stickleback (Gasterosteus aculeatus L. forma leiura Cuv.) in the Geelmolen brook near Vaassen has two reproductive periods: one in spring (first macro-breeding wave, I), and one in late summer (second macro-breeding wave, II). In general, only one reproductive period is known for the Stickleback, i.e., in spring. The phenomenon of macro-breeding wave II must be seen as an exceptional situation, possibly created by rich food supply and relatively high winter and moderate summer temperatures of the brook water. 2. Both macro-breeding waves show two micro-breeding waves (hatching-waves, I1, I2 and II1, II2). During a micro-breeding wave, all the eggs hatch on one given day ± a couple of days. The data make it seem probable that in 1963 the highest frequency of hatching for I1 fell on 11 May, for I2 on 2 June, for II1 on 3 August, and for II2 on 28 August. Outside these short periods, which include the dates mentioned ± a couple of days, no eggs hatch. It may therefore be stated that there is a high degree of synchronization in the reproduction of the Stickleback. 3. The interval between two micro-breeding waves amounts to about three weeks. 4. The interval between I2 and II1 amounts to about 2 months. 5. The growth of the young animals from the 4 micro-breeding waves during the period of the investigation (22 April through 17 October 1963) is shown by the growth lines in figure 2.