Conjugative elements often encode entry exclusion systems that convert host cells into poor recipients for identical or similar elements. The diversity of exclusion systems within families of conjugative elements has received little attention. We report here the most comprehensive study to date of the diversity of exclusion determinants within a single family of conjugative elements. Unexpectedly, our analyses indicate that there are only two exclusion groups among the diverse members of the SXT/R391 family of integrative conjugative elements.