  We report a study of Co-doped berthierite Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ ($x$ =
0, 0.1, and 0.2). The alloy series of Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ crystallize
in an orthorhombic structure with the Pnma space group, similar to FeSb$_2$,
and show semiconducting behavior. The large discrepancy between activation
energy for conductivity, $E_\rho$ (146 $\sim$ 270 meV), and thermopower, $E_S$
(47 $\sim$ 108 meV), indicates the polaronic transport mechanism. Bulk
magnetization and heat capacity measurements of pure FeSb$_2$S$_4$ ($x$ = 0)
exhibit a broad antiferromagnetic (AFM) transition ($T_N$ = 46 K) followed by
an additional weak transition ($T^*$ = 50 K). Transition temperatures ($T_N$
and $T^*$) slightly decrease with increasing Co content $x$. This is also
reflected in the thermal conductivity measurement, indicating strong
spin-lattice coupling. Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ shows relatively high value
of thermopower (up to $\sim$ 624 $\mu$V K$^{-1}$ at 300 K) and thermal
conductivity much lower when compared to FeSb$_{2}$, a feature desired for
potential applications based on FeSb$_{2}$ materials.
