  Recent calculations by Quataert et al. (2002) found that the growth rates of
the magnetorotational instability (MRI) in a collisionless plasma can differ
significantly from those calculated using MHD. This can be important in hot
accretion flows around compact objects. In this paper we study the transition
from the collisionless kinetic regime to the collisional MHD regime, mapping
out the dependence of the MRI growth rate on collisionality. A kinetic closure
scheme for a magnetized plasma is used that includes the effect of collisions
via a BGK operator. The transition to MHD occurs as the mean free path becomes
short compared to the parallel wavelength $2\pi/k_{\Par}$. In the weak magnetic
field regime where the Alfv\'en and MRI frequencies $\omega$ are small compared
to the sound wave frequency $k_{\Par} c_0$, the dynamics are still effectively
collisionless even if $\omega \ll \nu$, so long as the collision frequency $\nu
\ll k_{\Par} c_{0}$; for an accretion flow this requires $\nu \lsim \Omega
\sqrt{\beta}$. The low collisionality regime not only modifies the MRI growth
rate, but also introduces collisionless Landau or Barnes damping of long
wavelength modes, which may be important for the nonlinear saturation of the
MRI.
