  We report the observation of a pseudogap in the \emph{ab}-plane optical
conductivity of underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($x = 0.2$ and
0.12) single crystals. Both samples show prominent gaps opened by a spin
density wave (SDW) order and superconductivity at the transition temperatures
$T_{\it SDW}$ and $T_c$, respectively. In addition, we observe an evident
pseudogap below $T^{\ast} \sim$ 75 K, a temperature much lower than $T_{\it
SDW}$ but much higher than $T_{c}$. A spectral weight analysis shows that the
pseudogap is closely connected to the superconducting gap, indicating the
possibility of its being a precursor of superconductivity. The doping
dependence of the gaps is also supportive of such a scenario.
