Previous studies using the patch-clamp technique demonstrated the presence of a small conductance Cl channel in the apical membrane of respiratory gill cells in primary culture originating from sea bass Dicentrarchus labrax. We used the same technique here to characterize potassium channels in this model. A K⁺ channel of 123 ± 3 pS was identified in the cell-attached configuration with 140 mM KCl in the bath and in the pipette. The activity of the channel declined rapidly with time and could be restored by the application of a negative pressure to the pipette (suction) or by substitution of the bath solution with a hypotonic solution (cell swelling). In the excised patch inside-out configuration, ionic substitution demonstrated a high selectivity of this channel for K⁺ over Na⁺ and Ca²⁺. The mechanosensitivity of this channel to membrane stretching via suction was also observed in this configuration. Pharmacological studies demonstrated that this channel was inhibited by barium (5 mM), quinidine (500 µM), and gadolinium (500 µM). Channel activity decreased when cytoplasmic pH was decreased from 7.7 to 6.8. The effect of membrane distension by suction and exposure to hypotonic solutions on K⁺ channel activity is consistent with the hypothesis that stretch-activated K⁺ channels could mediate an increase in K⁺ conductance during cell swelling.