We initiate the study of computational problems on $k$-mers (strings of length $k$) in labeled graphs. As a starting point, we consider the problem of counting the number of distinct $k$-mers found on the walks of a graph. We establish that this is #P-hard, even on connected deterministic DAGs. However, in the class of deterministic Wheeler graphs (Gagie, Manzini, and Siren, TCS 2017), we show that distinct $k$-mers of such a graph $W$ can be counted using $O(|W|k)$ or $O(n^4 \log k)$ arithmetic operations, where $n$ is the number of vertices of the graph, and $|W|$ is $n$ plus the number of edges. The latter result uses a new generalization of the technique of prefix doubling to Wheeler graphs. To generalize our results beyond Wheeler graphs, we discuss ways to transform a graph into a Wheeler graph in a manner that preserves the $k$-mers. As an application of our $k$-mer counting algorithms, we construct a representation of the de Bruijn graph (dBg) of the $k$-mers in time $O(|dBg| + |W|k)$. Given that the Wheeler graph can be exponentially smaller than the de Bruijn graph, for large $k$ this provides a theoretical improvement over previous de Bruijn graph construction methods from graphs, which must spend $\Omega(k)$ time per $k$-mer in the graph. Our representation occupies $O(|dBg| + |W|k \log(\max_{1 \leq \ell \leq k}(n_\ell)))$ bits of space, where $n_\ell$ is the number of distinct $l$-mers in the Wheeler graph.

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