In recent years, the ties that Members of Parliament (MPs) create by cosponsoring legislation together have attracted interest from scholars adopting a network approach to lawmaking. This paper expands the empirical base of these studies by introducing an original dataset of 150 bill cosponsorship networks that cover 27 parliamentary chambers from 19 European countries, plus Israel. We then use the data to model the amount of partisan divisiveness found in the networks while controlling for the number of parliamentary factions, thereby providing an empirical test of Giovanni Sartori's classical distinction between the fragmentation of a party system and the degree of polarization between its components. In line with Sartori's observations, the results show that inter-party ideological distance predicts some of the partisanship expressed by MPs through their propensity to cosponsor bills within and across party lines, regardless of the number of MPs and parties involved.