Abstract: Short daylengths (SD) given after long daylength (LD) induce an immediate phase of growth rest followed by a synchronized resumption of growth after a more or less constant time period (10-20 weeks, depending on species) in sporophytes of Laminaria hyperborea (Gun.) Foslie, L. setchellii Silva and Pterygophora californica Ruprecht. Plants that are in a phase of rest when transferred from LD to SD resume growth after more or less the same defined time period than plants which are growing at the beginning of the experiment and their growth reaction is different from LD control plants. It is concluded that SD given after LD always induce a phase shift of the endogenous circannual rhythm to a phase of growth rest independent of the state of the plant activity at the time of transfer. Thus, SD seem to be the main synchronizing factor of the endogenous circannual growth rhythm in kelps. However, very young sporophytes less than about 3 months old do not stop growth in SD given after LD. This may indicate that the endogenous circannual clock is either not active in the first months after 'birth' or first has to develop during ontogeny.