The reproductive cycle of Coregonus lavaretus (L. ) in Loch Lomond was investigated by monthly sampling. The stages of the cycle occurred at the same time each year and were precisely timed. The environment followed a regular pattern which varied seasonally and was repeated annually. Spawning synchrony within the population during the short breeding period may be in response to lunar phases. A qualitative echosounding survey was made. The spatial distribution of the fish is probably related to their feeding behaviour and was mainly pelagic in summer and benthic in winter. Diel vertical migrations were recorded at dawn and dusk and appeared to be related to negative solar altitudes. The fish occurred at the surface during the night and persisted with their diel vertical migrations when feeding behaviour was benthic. The regulation of the reproductive cycle in the common sole Solea solea was investigated. The timing of spawning is ultimately determined by sea temperature, and spawning synchrony within the population is probably achieved in the initiation of exogenous vitellogenesis by a unified response within the breeding population to a stimulatory photoperiod. In both Solea solea and Coregonus lavaretus, initiation of exogenous vitellogenesis occurred during a rapid rise in the condition of the fish. The photosensitivity of the reproductive system may possibly be linked to a threshold condition. The pineal organ of Coregonus lavaretus is typically salmonid and the convoluted epithelium contained photoreceptors, interstitial cells, and neurones. The interstitial cells gave rise to processes which extended into the perivascular space. Photoreceptor cells synapsed with neurones, photoreceptor cells (lateral processes) and possibly other cell types. The results suggest that the pineal organ functions as a photoreceptor.