Until now, voltage-gated Ca2+ channel proteins have been found only in eukaryotes. Here we report that a gene recently discovered in the eubacterium Bacillus halodurans codes for a protein closely related to eukaryotic Ca2+ channels, but that has only one 6-transmembrane-segement (6TM) motif, instead of four, in its pore-forming subunit. This is supported by the comparison of consensus sequences, which, along with the patterns of residue conservation, indicates a similar structure in the membrane to voltage-gated K+ channels. From this we hypothesize that Ca2+ channels originally evolved in bacteria, and that the specific eubacteria protein highlighted here is an ideal candidate for structure determination efforts.