The reversal potential for the GABA-elicited hyperpolarization (EGABA) of the muscle bag cells of the parasite nematode Ascaris suum is near to the equilibrium potential for chloride ions (ECl) indicating that this is a chloride-mediated event. The value of EGABA in the presence of different anions indicates the selectivity sequence of the GABA-operated chloride channel as SCN- greater than Br-, I- greater than ClO3-, NO3-, Cl- greater than C2H5COO- greater than CH3COO-, HCOO-, OHCH2CH2SO3H-, CH3SO4-, BrO3-, SO4(2-). The cut-off point in terms of relative hydrated size for permeation is between ClO3- and BrO3- suggesting that the diameter of the channel is between 0.29 and 0.33 nm. The selectivity sequence of the resting membrane chloride channels was indicated by the degree of muscle cell hyperpolarization upon addition of the anion to the bathing medium and was SCN- greater than NO3- greater than I-, Br-, greater than ClO3- greater than Cl-. These sequences are consistent with the anions interacting with a cationic site of low field strength at the selectivity determining portion of the channel for both the GABA-operated and the resting chloride channel.