Potassium channels govern the permeability of cells to potassium ions, thereby controlling the membrane potential. In metazoa, potassium channels are encoded by a large, diverse gene family. Previous analyses of this gene family have focused on its diversity in mammals. Here we have pursued a more comprehensive study in Caenorhabditis elegans, Drosophila melanogaster, and mammalian genomes. The investigation revealed 164 potassium channel encoding genes in C. elegans, D. melanogaster, and mammals, classified into seven conserved families, which we applied to phylogenetic analysis. The trees are discussed in relation to the assignment of orthologous relationships between genes and vertebrate genome duplication.