Résumé
This paper deals with the Latin thematic genitive in Indo-European perspective. The origin of the ending -ī has been disputed since the beginnings of Indo-European linguistics because it shows no trace of the thematic vowel. Firstly, the comparative data are presented since the ending -ī is widespread through several Western Indo-European languages (Faliscan, Venetic and Celtic, except for Celtiberian); moreover, it has been recently detected in Tokharian. Following the scenario put forward by Pinault (forthcoming), it is then argued that the ending -ī can be traced back to the neuter of relational adjectives in *-ih2, which stand by the thematic declension according to an archaic pattern.