Flame propagation of lower limit hybrid coal dust-methane-air mixtures in vertical and horizontal flame ducts of 15.2 cm id and 1.8 m length has been investigated to provide information bearing on safety in mines. Fuel concentrations at lower limits of flame propagation and equivalences of coal dust and methane are much higher for continuing flame propagation than for short-span flame propagation in the vicinity of an ''overdriving'' ignition source. As the energy of the ignition source was increased, the flame speed and the distance of flame propagation from the ignition source increased, whereas the total fuel concentration at the lower limit decreased.