The article aims to explain voting unity in the Chamber of Deputies of the Parliament of the Czech Republic based on data from the years 1998–2002. It introduces the basic terminology and theoretical framework used in literature on the behaviour of parties in parliament and the basic institutional rules that should result in the unity of political parties in the Czech parliament. It then presents the data used to measure the unity of Czech parliamentary party groups. The initial assumption that specific institutional factors found in parliament and in political parties would lead to greater PPG unity in the Czech Republic was not confirmed. Although the institutional incentives are similar to those in Western European countries, they do not secure the same level of voting unity in Czech PPGs. The authors conclude that the relatively low party unity is caused by the size of the voting coalitions that pass individual bills.