The growth rate of the collisional tearing instability is calculated for a current sheet with a low magnetic Lundquist number S. The maximum growth rate is found to be &ggr;m ≂0.075tA-1 tanh (8/S0.5) for 3≤S≤105, where tA is the Alfv¿n transit time in the current sheet. For S>300, the growth rate can be written approximately as &ggr;m ≂0.6tA-1 S0.5; for S<50, the growth rate approaches an asymptotic value of ~0.075tA-1. The present result is applicable to the current sheet associated with the diffusion region of a magnetic reconnection configuration, in which the Lundquist number, based on the thickness of the diffusion region, is typically 2≤S≤100.