The two pore domains of tandem pore K⁺ channels lack sequence identity. The pH dependence of TASK-1 is at least partly regulated by protonation of the side chain of a histidine residue (H98) which lies just beyond the consensus sequence for K⁺ selectivity in P1 (Ashmole et al., 2001). In P2 the equivalent residue is an aspartate (D204). We mutated both residues in turn, substituting both the H and D for an asparagine (N).Murine TASK-1 channels were expressed in oocytes taken from Xenopus frogs that had been anaesthetised by immersion in 0.3% w/v MS222 and killed by destruction of the brain and spinal cord. We used two-electrode voltage clamp to investigate ionic selectivity and pH sensitivity. Ionic selectivity was calculated by measuring the shift in reversal potential when K⁺ in the external medium was replaced by Rb⁺ or Na⁺.We have previously shown that the mutant H98D exhibits reduced pH sensitivity and ionic selectivity, and also that protonation of a single His is sufficient for channel closure (Yuill et al., 2003). Here we show that the residue D204 in P2 also plays a role in pH sensitivity and is required for ionic selectivity. The mutant D204N was found to be largely insensitive to changes in pH over the physiological range, where substantial currents could be recorded. Currents increase further under alkaline conditions, with a pK_a of 8.26 at +40mV. These results complement our previous findings for a D204H mutation and confirm the importance of D204 in pH sensitivity. H98N exhibited reduced pH sensitivity compared to wild type and also to H98D channels. Fitting the residual pH sensitivity gave a pK_a of 6.89 at +40mV (n=7).The mutants H98N and D204N also altered channel selectivity. In oocytes expressing wild type channels the P_Rb/P_K was 0.77 ± 0.02 (n=6). For H98N the P_Rb/P_K of 0.81 ± 0.03 (n = 7) was not significantly different to wild type, but D204N was significantly increased to 0.97 ± 0.01 (n = 3; P<0.01). Wild type channels discriminate well against sodium, giving a P_Na/P_K of 0.02 ± 0.003. However, channels became significantly more Na⁺ permeant after mutation. The P_Na/P_K increased to 0.21 ± 0.04 (n = 7) for H98N, and 0.33 ± 0.07 (n = 3) for D204N (P<0.01). Thus, D204 is also required for ionic selectivity. We find that two-fold symmetry best supports both pH sensitivity over the physiological range and K⁺ selectivity.