By using immunohistochemical S-ABC technique, AChE and M₁ receptor in the embryos and larvae of Arenicola brasiliensis were detected.The results showed that AChE and M₁ receptor were positivity stained from unfertilized eggs to 5-setiger larvae.However, their distribution was distinctive at different phases.For example, at the cleavage stage AChE and M₁ receptors were mainly distributed in the plasma membrane and perinuclear cytoplasm, while at the larval stage they were mainly detected in the head region, digestive tract, the areas where cilia grew, and setae etc.Furthermore,AChE and M₁ receptors had a similar distribution pattern in embryos and larvae.The results indicate that ACh may act as an important regulator during embryo and larvae development.