  In the first part of the paper we describe $\varphi$-derivations of the
incidence algebra $I(X,K)$ of a locally finite poset $X$ over a field $K$,
where $\varphi$ is an arbitrary automorphism of $I(X,K)$. We show that they
admit decompositions similar to that of usual derivations of $I(X,K)$. In
particular, the quotient of the space of $\varphi$-derivations of $I(X,K)$ by
the subspace of inner $\varphi$-derivations of $I(X,K)$ is isomorphic to the
first group of certain cohomology of $X$, which is developed in the second part
of the paper.
