Although the regulation of events in the cell division cycle by calcium or other cations has been the subject of much interest and speculation, experimental studies have been hampered by the difficulty of measuring submicromolar intracellular free calcium concentrations ([Ca2+]i) over an entire cell cycle. We now describe experiments using a new fluorescent calcium chelator, fura-2 (see Fig. 1c for structure), for continuous measurement of [Ca2+]i from fertilization through the first cleavage of individual eggs of the sea urchin Lytechinus pictus. We also show for comparison the results of parthenogenetic activation by ammonia. In addition to the known transient rise of [Ca2+]i at fertilization, further peaks are now revealed during pronuclear migration, nuclear envelope breakdown, the metaphase/anaphase transition and cleavage. Parthenogenetic activation by ammonia also elicits a sustained rise starting at nuclear envelope breakdown.