Résumé : The study of political parties and parliaments has for a long time been dominated by a central assumption: parties act as homogenous and unified actors. But recently, scholars have opened the ‘black-box’ of parties and have given growing attention to intra-party dynamics. During the last decade, legislative scholars have examined the extent to which and the reasons why parliamentarians would vote against their party line, and have found that such ‘rebellions’ were quite rare in European legislatures. Yet the reasons for such unity remain obscured: do parties arrived in parliament as cohesive blocks, or are MPs disciplined by institutional constraints? This paper addresses the issue of cohesion within European parliamentary parties regardless of voting unity scores, by examining parliamentarians’ frequency of disagreement with their party on the basis of the Comparative PARTIREP MP Survey database. In this way, the paper shows that European parliamentary parties in fact vary quite importantly as far as their degree of ‘pre-floor’ agreement is concerned. The paper then looks at two explanatory factors of cohesion, and examines their combined effect: on the one hand, the parliamentarians’ ideological positioning; and on the other hand, the party’s ideology –i.e. a factor that has rarely been investigated as a factor of legislative (dis)unity. The paper analyzes these relationships at both the individual (MP) and aggregate (party) levels.