  We solve the linearized gyrokinetic equation, quasineutrality condition, and
Ampere's law to obtain the dispersion relation of microtearing modes (MTMs) in
collisionless low-beta toroidal plasmas. Consistent with past studies, we find
that MTMs are driven unstable by the electron temperature gradient and that
this instability drive is mediated by magnetic drifts. The dispersion relation
that we derive can be evaluated numerically very quickly and may prove useful
for devising strategies to mitigate MTM instability in fusion devices.
