The gonadal index cycle of the echinoid Strongylocentrotus droebachiensis is described from observations on the Atlantic and Pacific coasts of Canada. An annual cycle, with spring spawning, is evident, but the exact timing of spawning and of gonadal growth varies. Consideration is given to controlling factors, using information from the present and previous studies. A number of gonadal events begin abruptly, which suggests they are controlled externally. An increase in energy channeled into gonadal growth in midsummer may be stimulated by an environmental cue. However, advanced gonadal growth will occur when excess food is available, and none will occur if food is not available. Spawning is not closely correlated with temperature, but records from the North Pacific, Arctic, and North Atlantic Oceans show that spawning is synchronized with the spring phytoplankton increase. An experimental study has confirmed that phytoplankton is the spawning cue. Concurrent observations of Strongylocentrotus purpuratus show a prolonged spawning, compared with S. droebachiensis, suggesting a different mechanism initiating spawning.