To predict origins of replication in prokaryotic chromosomes, we analyse the leading and lagging strands of 200 chromosomes for differences in oligomer composition and show that these correlate strongly with taxonomic grouping, lifestyle and molecular details of the replication process. While all bacteria have a preference for Gs over Cs on the leading strand, we discover that the direction of the A/T skew is determined by the polymerase-α subunit that replicates the leading strand. The strength of the strand bias varies greatly between both phyla and environments and appears to correlate with growth rate. Finally we observe much greater diversity of skew among archaea than among bacteria. We have developed a program that accurately locates the origins of replication by measuring the differences between leading and lagging strand of all oligonucleotides up to 8 bp in length. The program and results for all publicly available genomes are available from