This article tests the thesis that the internal agential characteristics of the Green party segment in a relatively open and dynamic party system are a crucial factor in the long–term success or failure of Green parties in that system. The case study of Slovenia offers a natural laboratory to test this thesis, because its political system is characterised by institutions and a party system that are favourable to the establishment of new parties and by a persistently relatively high environmental consciousness among voters. In such a context, internal party developments obtain critical importance for explaining the decline of the Green party segment in Slovenia.