Let $k$ be a positive integer, and let $G$ be a simple graph with vertex set $V(G)$. A {\it $k$-dominating set} of the graph $G$ is a subset $D$ of $V(G)$ such that every vertex of $V(G)-D$ is adjacent to at least $k$ vertices in $D$. A {\it $k$-domatic partition} of $G$ is a partition of $V(G)$ into $k$-dominating sets. The maximum number of dominating sets in a $k$-domatic partition of $G$ is called the {\it $k$-domatic number} $d_k(G)$. \endgraf In this paper, we present upper and lower bounds for the $k$-domatic number, and we establish Nordhaus-Gaddum-type results. Some of our results extend those for the classical domatic number $d(G)=d_1(G)$.