Propulsion of the spermatozoon depends on the precise coordination of a propagated flagellar wave. True acetylcholinesterase, which is concentrated in the flagellum, may regulate the motility by controlling the intracellular level of acetylcholine. Spermatozoa oriented in a centrifugal field showed a strong biphasic response to added eserine, diisopropylfluorophosphate and ouabain; higher concentrations of those agents (100 to 1000µ M and above) slowed sperm migration while lower concentrations increased sperm motility above the control rate. Strong positively charged quaternary ammonium compounds, tetraethylammonium, neostigmine and acetylcholine had only slight effect on the swim rate, presumably because of their inability to penetrate the sperm plasma membrane. Potassium chloride added to the filtered sea water slowed sperm motility at all concentrations tested, causing 50% inhibition when added in an equivalent amount to that already present in sea water (9 mM). Arbacia sperm motility determined by the change in optical density of centrifugally oriented suspensions averaged 187 µm/sec.