  Let $k$ be a perfect field of characteristic $p > 0$, and let $K = k((u))$ be
the field of Laurent series over $K$. We study the skew polynomial ring $K[T,
\Phi]$, where $\Phi$ is an endomorphism of $K$ that extends a Frobenius
endomorphism of $k$. We give a description of the irreducible skew polynomials,
develop an analogue of the theory of the Newton polygon in this context, and
classify the similarity classes of irreducible elements.
