Crystals of the heterocyclic radical naphtho1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at T-c down arrow = 128(2) K and T-c up arrow = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent pi-radicals in the PI unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (S = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N-N sigma-bonded dimers (S = 0) and half retain their S = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition.