Plasma membrane vesicles prepared from the bag region of the somatic muscle cells of the parasitic nematode Ascaris suum contain high conductance, voltage sensitive, Ca-dependent chloride channels, suggested to be involved in the excretion of carboxylic acids produced by the anaerobic respiration of glucose (Valkanov, Martin & Dixon, 1994). The effect of altered pH on this channel was investigated using the patch-clamp technique and isolated inside-out membrane patches. Changes in pH had little effect on channel conductances and only a small effect on reversal potentials. Under control conditions (symmetrical pH 7·2) the channel had the highest probability of opening at ∼ — 35 mV (the resting membrane potential of the cell). At positive membrane potentials the probability of opening decreased. The Boltzmann equation was used to describe the relationship between membrane potential and probability of channel opening, and to calculate the effective gating charge. Reduction of external pH produced an increase in the probability of channel opening at hyper-polarized membrane potentials. An increase in internal pH caused a voltage-independent increase in the probability of channel opening and made the effective gating charge less negative. The effect of reducing internal pH was marked: the channel then opened most frequently at positive membrane potentials and the probability of opening at —35 mV was greatly reduced. The decrease in internal pH changed the polarity of the effective gating charge. A simple model was constructed to describe the effects of pH on channel gating.