To ensure that citizen suits assist but do not replace or overshadow government enforcement actions, all environmental statutes which authorize citizen suits bar such suits in certain circumstances. This short Article examines the relatively narrow but important problems created by one such bar, namely the statutory bar on a citizen suit if the federal or state government is “diligently prosecuting” an action against the same violator. The requirement that a governmental prosecution be diligent protects against two types of undesirable situations. On the one hand, the diligent prosecution bar prevents citizens from bringing simple “me too” actions. One would not want to encourage a citizen organization to wait until a government has investigated and worked up a case and then pile on the violator in the hopes of gaining money for wasteful pet projects and collecting attorneys' fees. On the other hand, by requiring that a prosecution be diligent (and not merely in existence) before the bar applies, the statutes protect valid citizen suits from being wiped out by collusive action between a government and a violator brought specifically by the government to bar the citizen suit and force the citizens to lose the resources they have sunk into the case. Obviously, most cases involving concurrent citizen and government enforcement actions fall somewhere between these two extremes. The goal of this Article is to find a balance between these two ends and to explore means of determining just how diligent a prosecution must be to bar citizen enforcement. In examining this question, I will pay special attention to the diligent prosecution bars in the Clean Water Act because they are the most comprehensive but I will make reference to cases from other environmental statutes as well.