Molecular dynamics and umbrella sampling calculations are used to explore extracellular blockade of the KcsA by tetraethylamonium (TEA), a classical blocker of K+ channels. It is found, in remarkable agreement with experiment, that TEA is more stably bound when there are aromatic residues at position 82. The enhanced stability conferred by the aromatic residues does not arise from Pi-cation interactions as was previously suggested, but is due to hydrophobic interactions.