The motile TnphoA mutant IC24 of Proteus mirabilis U6450 generates an aberrant swarming colony, and was shown to be impaired in swarm cell differentiation, i.e. cell elongation and hyperflagellation, causing delayed and slower population migration across a solid growth medium. Levels of transcript from the flagellin filament gene fliC, the flagellar master operon flhDC, and the leucine-responsive regulatory protein gene lrp, a regulator of swarming differentiation, were reduced in IC24 mutant swarm cells. The transposon had inserted into a gene encoding a putative P-type ATPase closely related to those transporting cations across bacterial membranes. This ppa gene (Proteus P-type ATPase) was maximally expressed in differentiated swarm cells. The data suggest an effect of ion homeostasis on swarm cell differentiation, possibly mediated via the lrp-flhDC pathway.