1. The anal papillae of the aquatic larva of Aëdes aegypti are responsible for 90% of the steady-state exchange of chloride.
2. The relationships between chloride flux and external chloride concentration are approximately described by the Michaelis equation.
3. There is net uptake of chloride, independent of uptake of sodium, from KCl, CaCl2 and NH4Cl, probably in exchange for OH' or HCO'3, but the rate is much slower than from NaCl. The following ions stimulate influx of chloride from 0.1 mM/l. KCl: H+ = Na+ < K+ The following ions inhibit it: OH' > HCO'3 > NO'3.
4. Movements of sodium and chloride ions are explicable in terms of an anionic and a cationic carrier located in an osmotic barrier in the papillae, the carriers being functionally coupled to sodium and chloride pumps located at the inner surface of the barrier.
5. An attempt is made to relate these findings to recent electron microscopical studies of the papillae.