campestris ~ ~~Three-hundred and twenty-six strains of Xanthomonas campestris pv. manihotis from 22 countries were studied to detect and assess genetic and evolutionary relationships within the pathovar. A range of techniques was used for this study including restriction fragment length polymorphism (RFLP) analysis. The probes used for the RFLP analysis were 16 + 23s rRNA genes from Escherichia coli and three restriction fragments from the chromosomal and plasmid DNA of X. campestris pv. manihotis. Analysis of the rRNA probe data showed five RFLP groups whilst the other three probes were used to further sub-divide these groupings. Genetic variability of X. campestris pv. manihotis was pronounced in strains from South America where the host plant originated but was limited in strains from other regions. The results obtained confirm the hypothesis that the pathogen has been introduced only recently to Africa and suggests that African strains have not as yet diversified significantly at the chromosomal level. Our results indicate that rRNA and DNA probes are useful tools for epidemiological studies and in following the genetic evolution of strains.