The isolated midgut of the larvae of the American silkworm transports potassium from blood-side to lumen, when the bathing solution contains potassium, magnesium, calcium and sucrose. When potassium is substituted by caesium the midgut transports caesium. The competition between potassium and caesium for the transport mechanism is unusual-with more than 30% potassium no caesium will be transported, with less than 20% no potassium will be transported. Sodium, lithium and ammonium are not transported under these conditions. When magnesium and calcium are removed from the solutions also lithium, sodium and ammonium are transported. Potassium competes equally with rubidium for the transport mechanism, but in competition with the rest of the alkali metals potassium is the preferred ion. With 50% potassium in the solution only potassium will be actively transported.