Sperm flagellar motility is activated and regulated by egg factors at fetilization.  In the ascidian Ciona intestinalis, a sulfated steroid called SAAF (sperm activating and attracting factor) induces both sperm motility activation and chemotaxis¹ .  Cyclic AMP (cAMP) is one of the most important intracellular factors in the signaling pathway for SAAF-induced sperm activation.  Adenylyl cyclase (AC) is the key enzyme that synthesizes cAMP at the onset of the signaling pathway to activate axonemal dyneins. In Ciona sperm it is reported that a 21 kDa light chain of outer arm dynein (LC2) and a 26 kDa axonemal protein are phosphorylated, and dynein intermediate chains IC2 and IC116 are dephosphorylated in a cAMP-dependent manner²³ .  We  previously reported that both transmembrane AC (tmAC) and soluble AC (sAC) are present and function in Ciona sperm but tmAC play a major role in SAAF-induced motility activation.  In this study we focused on the role of sAC in the regulation of flagellar motility in Ciona sperm.  We demonstrated that a sAC inhibitor did not affect on SAAF-induced transient increase of cAMP or motility activation, but it reduced swimming velocity to half and increased the swimming path curvature.  In addition sAC inhibitor significantly blocked sperm chemotaxis toward SAAF.  These results indicate that sAC critically plays a role distinct from tmAC in the regulation of flagellar motility such as an increase in flagellar beat frequency and Ca²⁺-dependent regulation of flagellar waveform during chemotactic movement.