The Potentiometric Cycling for Polymorph Creation [PC] 2 has been applied to sparfloxacin, a third-generation fluoroquinolone antibiotic. Two different trihydrate phases precipitate from aqueous solution. X-ray data indicate that one of these is a previously unknown polymorph of sparfloxacin trihydrate. Because both forms crystallize from solution at the same time, the two crystalline forms are concomitant polymorphs that precipitate in a thermodynamically controlled ratio.