A directed dominating set in a directed graph D is a set S of vertices of V such that every vertex u is an element of V (D) \ S has an adjacent vertex v in S with v directed to u. The directed domination number of D, denoted by gamma(D), is the minimum cardinality of a directed dominating set in D. The directed domination number of a graph G, denoted by Gamma(d)(G), is the maximum directed domination number gamma(D) over all orientations D of G. The directed domination number of a complete graph was first studied by Erdos [P. Erdos, On Schutte problem, Math. Gaz. 47 (1963) 220-222], albeit in disguised form. The authors [Y. Caro, M.A. Henning, A Greedy partition lemma for directed domination, Discrete Optim. 8 (2011) 452-458] recently extended this notion to directed domination of all graphs. In this paper we continue this study of directed domination in graphs. We show that the directed domination number of a bipartite graph is precisely its independence number. Several lower and upper bounds on the directed domination number are presented. (C) 2012 Elsevier B.V. All rights reserved.