The emergence in several countries of the monophasic serogroup D1 serovar Salmonella 9,12:l,v:- provided the opportunity to study its evolutionary origin. According to current models, such a variant serovar could have arisen by horizontal transfer of a new flagellar gene to a preexisting monophasic Salmonella strain or, alternatively, by the loss of the phase 2 flagellar gene of an originally biphasic Salmonella strain. Five known serovars of Salmonella, S. panama, S. kapemba, S. goettingen, S. zaiman, and S. mendoza, could have been possible ancestors of the new variant. The profiles of the insertion element IS200, which has been shown to provide phylogenetic markers for serogroup D1 salmonellae, were analyzed in relation to the restriction fragment length polymorphisms of the phase 2 flagellar gene. Together they provide unequivocal evidence that Salmonella 9,12:l,v:- arose from a strain of S. goettingen. Analysis of the flj operon of the variant indicated that loss of phase 2 flagellar antigen expression occurred through deletion of the hin gene and adjacent DNA, thereby blocking the phase 2 flagellar gene in the off position.