In this chapter we analyze the internal ideological homogeneity of Belgian parties and we compare it with that of parties in three other countries: Germany, Switzerland and Canada. None of these other countries is considered to be a partitocracy; the Swiss case is sometimes even described as the exact opposite, namely a system that does not entail party government and with relatively weak parties. We employ novel data drawn from a survey of 851 politicians-MPs, ministers and party leaders-in these four countries. Our questionnaire included eight (or nine in Switzerland) policy proposals that politicians were asked their personal opinion about. This allows to calculate ideological agreement on the country, party and individual level. Apart from using objective opinion ‘distance’ between a partisan and his/her party as a measure of ideological agreement, we also use questions in which politicians were asked to make a subjective estimation of how often they disagree with their party compared to their colleagues. We find that parties in all four countries are rather ideologically homogeneous. On average, individual politicians agree with their party’s stance (i.e. with the majority of MPs in their party) on four out of five policy proposals. There are only small differences between individual representatives but much larger differences across parties. In the four countries under study, government parties, left-wing parties and populist parties are more internally homogenous (they have less members who diverge from the party line on several policy proposals) than opposition, right-wing and non-populist parties. Further, we find that politicians diverge from their party especially with regard to issues on which they perceive their electorate to be at odds with what the party wants. Most importantly, the country in our sample with the weakest parties, Switzerland, also appears to have the least ideologically homogenous parties; many party representatives in Switzerland disagree with the majority of their partisan ...