The pteropod mollusk Clione limacina swims by dorsal-ventral flapping movements of its wing-like parapodia. Two basic swim speeds are observed—slow and fast. Serotonin enhances swimming speed by increasing the frequency of wing movements. It does this by modulating intrinsic properties of swim interneurons comprising the swim central pattern generator (CPG). Here we examine some of the ionic currents that mediate changes in the intrinsic properties of swim interneurons to increase swimming speed in Clione. Serotonin influences three intrinsic properties of swim interneurons during the transition from slow to fast swimming: baseline depolarization, postinhibitory rebound (PIR), and spike narrowing. Current clamp experiments suggest that neither I_h nor I_A exclusively accounts for the serotonin-induced baseline depolarization. However, I_h and I_A both have a strong influence on the timing of PIR—blocking I_h increases the latency to PIR while blocking I_A decreases the latency to PIR. Finally, apamin a blocker of I_K(Ca) reverses serotonin-induced spike narrowing. These results suggest that serotonin may simultaneously enhance I_h and I_K(Ca) and suppress I_A to contribute to increases in locomotor speed.