  We study the low-temperature electrical and thermal conductivity of CoSi and
Co$_{1-x}$M$_x$Si alloys (M = Fe, Ni; $x \leq$ 0.06). Measurements show that
the low-temperature electrical conductivity of Co$_{1-x}$Fe$_{x}$Si alloys
decreases at $x > $ 0.01 by an order of magnitude compared with that of pure
CoSi. It was expected that both the lattice and electronic contributions to
thermal conductivity would decrease in the alloys. However, our experimental
results revealed that at temperatures below 20K the thermal conductivity of Fe-
and Ni-containing alloys is several times larger than that of pure CoSi. We
discuss possible mechanisms of the thermal conductivity enhancement. The most
probable one is related to the dominant scattering of phonons by charge
carriers. We propose a simple theoretical model that takes into account the
complex semimetallic electronic structure of CoSi with nonequivalent valleys,
and show that it explains well the increase of the lattice thermal conductivity
with increasing disorder and the linear temperature dependence of the thermal
conductivity in the Co$_{1-x}$Fe$_x$Si alloys below 20K.
