  The model of Kondo chain with $M$-fold degenerate band of conduction
electrons of spin 1/2 interacting with localized spins $S$ is studied for the
case when the electronic band is half filled. It is shown that the spectrum of
spin excitations in the continuous limit is described by the O(3) nonlinear
sigma model with the topological term with $\theta = \pi(2S - M)$. For a case
$|M - 2S| = $(even) the system is an insulator and single electron excitations
at low energies are massive spin polarons. Otherwise the density of states has
a pseudogap and vanishes only at the Fermi level. The relevance of this picture
to higher dimensional Kondo insulators is discussed.
