Chloride transport in unionid bivalves is cation independent and is apparently an anion exchange system. The influx of Cl is highly correlated with the appearance of titratable base in the bathing medium. There is a substantial exchange diffusion component of Cl fluxes in these mussels. The influx of Cl displays saturation kinetics with a maximum velocity of 1 μEq (g dry tissue·h)⁻¹ and an affinity of 0.1 mM Cl/liter. Salt depletion doubles the rate of Cl influx by increasing the active transport component, with no change in the affinity. Chloride uptake is inhibited about 80% by 2 mM/liter thiocyanate but is not affected by acetazolamide or furosemide.