In the limit where the electron drift-wave frequency exceeds theelectron-ion collision frequency, drift-tearing modes are found to growwith a linear growth rate independent of resistivity and proportional tothe product of the electron inertial skin depth and the ion sound Larmorradius. The stabilization of these modes in collisionless andsemi-collisional regimes is investigated. The stabilization mechanism isrelated to the coupling and propagation of drift-acoustic perturbationsaway from the reconnecting mode-rational surface. Analytic and numericalsolutions of the four-field reduced fluid model in the slab geometryapproximation with constant electron temperature and negligible iontemperature are presented. The actual stability threshold can occur atvalues of the normalized tearing mode stability parameter Δ' ashigh as 102.\pacs{52.35.P}}\fnm{1}{Permanent address: Istituto Nazionale Fisica della Materia, Department of Energetics, Politecnico di Torino, Italy