Analysis of genetic variation between the banana and the citrus races of R. similis by starch gel electrophoresis demonstrated that 7 of 16 enzyme-encoding loci could be used for their diagnostic separation. The 2 races are closely related and share approximately 75% of the enzymes evaluated. The level of dissimilarities of inherited bands indicates that no gene flow occurs between the races. Aldolase, .alpha. + .beta. esterase, glucose-6-phosphate dehydrogenase, isocitrate dehydrogenase, lactate dehydrogenase, malate dehydrogenase and phosphoglucose isomerase are diagnostic markers of the races.