Matured oocytes of the annelidan worm Pseudopotamilla occelata are fertilized at the first metaphase of the meiotic division. During the activation by fertilizing spermatozoa, the mature oocyte shows a two-step intracellular Ca2+ increase. Whereas the first Ca2+ increase is localized and appears to utilize the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ stores, the second Ca2+ increase is global and involves Ca2+ influx via voltage-gated Ca2+ channels on the entire surface of the oocyte. To study how sperm trigger the Ca2+ increases during fertilization, we prepared soluble sperm extract (SE) and examined its ability to induce Ca2+ increases in the oocyte. The SE could evoke a Ca2+ increase in the oocyte when it was added to the medium, but not when it was delivered by microinjection. However, the second-step Ca2+ increase leading to the resumption of meiosis did not follow in these eggs. Local application of SE induced a non-propagating Ca2+ increase and formed a cytoplasmic protrusion that was similar to that created by the fertilizing sperm at the first stage of the Ca2+ response, important for sperm incorporation into the oocyte. Our results suggest that the fertilizing spermatozoon may trigger the first-step Ca2+ increase before it fuses with the oocyte in a pathway that involves the G-protein-coupled receptor and phospholipase C. Thus, the first phase of the Ca2+ response in the fertilized egg of this species is independent of the second phase of the Ca2+ increase for egg activation.