Bacteria exchange genes rarely but are promiscuous in the choice of their genetic partners. Inter-specific recombination has the advantage of increasing genetic diversity and promoting dissemination of novel adaptations, but suffers from the negative effect of importing potentially harmful alleles from incompatible genomes. Bacterial species experience a degree of`sexual isolation' from genetically divergent organisms^recombination occurs more frequently within a species than between species. In this review, I outline the sources and mechanisms of sexual isolation within the context of selective pressures acting on different types of recombination events. ß