This article extends the analysis of political parties in electorally volatile and organizationally weak party systems by evaluating two implications centered on legislative voting behavior. First, it examines whether disunity prevails where weakness of programmatic and electoral commonalities abound. Second, it analyzes whether inchoate party systems weaken the ability of government parties to control the congressional agenda. The empirical analysis centers on Peru, a classic example of a weakly institutionalized party system, and how its legislative parties compare to those of Argentina, Brazil, Chile, and the United States. The results lend support to the view that lower unity characterizes weakly institutionalized settings. The agenda-setting power of government parties, however, appears to be influenced more by the majority status of the government than by the level of party system institutionalization.