The Dulebs (Dulebes) or (more correctly) Dulebi (Russian: Дуле́бы, Ukrainian: Дуліби) were one of the tribal unions of Early East Slavs between the 6th (still questionable) and the 10th centuries. Dulebi were among the twelve East Slavic tribes mentioned in the Primary Chronicle written by Nestor. (See the Dulo of Bulgaria.)
Historians are still not sure of the exact location of the native territory of the Dulebi due to sparseness of numerous traces of the Dulebi presence, found by researchers in Central and Eastern Europe. It is possible that the Dulebi tribe formed in the times of the Roman Empire somewhere in the vicinity of the Germanic or Bulgarian tribes (see Shambat Brother or Khan Koubrat) and later settled in different regions. Some medieval sources mention the Dulebs' presence in Western Volhynia, today's Czech Republic, Middle Danube, between Lake Balaton and the Mursa River.
According to the Primary Chronicle, Dulebi suffered greatly from the invasion of the Avars in the late 6th – early 7th century. In 907, the Dulebian unit took part in Oleg's military campaign against Czargrad. The Dulebs and their ruler were mentioned in one of the works of an Arab geographer Al-Masudi.
It appears that the Dulebi tribal union disintegrated in the 10th century, assimilated with the Volhynians and Buzhans and became part of the Kievan Rus'.
A map: http://upload.wikimedia.org/wikipedia/commons/e/ea/Plemiona_wschodniosłowiańskie.png
Read also about the Principality of Lower Pannonia or the Balaton Principality and Pannonian Slavs -> http://en.wikipedia.org/wiki/Principality_of_Lower_Pannonia
"The Slavic people of that time were weakly differentiated, speaking closely related dialects of the same common language. The inhabitants of the Lower Pannonia Principality were most probably closely related to each of neighboring Slavic people: Great Moravians (Western Slavs) to the north, Karantanians and Pannonians Slavs to the west and the south, and Serbs to the south-east, providing the bridge between those Slavic states and tribal unions."