We show that dwarf nova disks in quiescence have rather low magnetic Reynolds number, of order 10^3. Numerical simulations of magnetized accretion disks suggest that under these conditions magnetohydrodynamic turbulence and the associated angular momentum transport is sharply reduced. This could be the physical origin of episodic accretion in dwarf nova disks. If so, the standard disk instability model needs to be revised.

4 pages, 2 postscript figures, Latex, uses emulateapj.sty. To be published in Ap. J. Letters