All living organisms fall into discrete clusters of closely related individuals on the basis of gene sequence similarity. Evolutionary genetic theory predicts that in the bacterial world, each sequence similarity cluster should correspond to an ecologically distinct population. Indeed, surveys of sequence diversity in protein-coding genes show that sequence clusters correspond to ecological populations. Future population surveys of protein-coding gene sequences can be expected to disclose many previously unknown ecological populations of bacteria. Sequence similarity clustering in protein-coding genes is recommended as a primary criterion for demarcating taxa.