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The control of annual rhythmicity in Agonum assimile was investigated by dissections and by breeding experiments both with specimens captured in the field and these reared in the laboratory. A. assimile is a spring breeder with summer larvae. The beetles hatch from the pupa from July until September, hibernate and breed in the following spring from April until June. Only a part of the old generation dies during the winter. Beetles from the previous year form about half of the breeding population. Rearing to the adulthood under laboratory conditions is possible with a success of about 80-90%, depending on the quality of food. Maturation of the sexes in the laboratory is desynchronized when the beetles are kept at 20°C and under "long day" conditions; development of the males is delayed in this case. In both sexes maturation takes place potentially free of dormancy.Development of the female and male gonads is mainly under the control of photoperiod and retarded in different ways by decreasing day length (photoperiodic quiescence). Thereby a maturation of the females is hindered in the autumn, and slow, further development of the males during the winter is made possible. The termination of quiescence by the transition from the "short day" to "long day" in laboratory experiments, or by increasing day length in the spring, synchronizes complete development in both sexes. The females normally produce an offsping of 20-60 larvae. We succeeded in keeping laboratory bred specimens up to 3 years, over this span they could be brought to propagation several times.