  The influence of disorder on the transmission through periodic waveguides is
studied. Using a canonical form of the transfer matrix we investigate
dependence of the Lyapunov exponent $\gamma$ on the frequency $\nu$ and
magnitude of the disorder $\sigma$. It is shown that in the bulk of the bands
$\gamma \sim \sigma^2$, while near the band edges it has the order $\gamma \sim
\sigma^{2/3}$. This dependence is illustrated by numerical simulations.
