The aim of this study was to explore the ruminal microbiota of lambs genetically extreme for their feeding speed when eating concentrates. From a database of 1001 lambs phenotyped for their feeding behavior, we estimated breeding values for their feeding speed at the visit level, and selected amongst 77 lambs phenotyped, 20 extreme lambs : 10 “high feeding speed” lambs with an average value of 57.8 g/min and 10 “low feeding speed” lambs with an average value of 26.8 g/min. The ruminal microbiota of these animals was profiled – after a first period of concentrates diets then a second period of forage – based diets - using 16S rRNA gene sequencing were analysed with FROGS software leading to 228 OTUs with taxonomic affiliation and characterized by their relative abundances. Microbiota differences was mainly due to the type of feed ingested-with higher bacteroidetes abundances when lambs ingested concentrates-since 211 of the 228 OTUs have abundance differences with respect of diet. Only 6 OTUs were significantly different according to feeding speed of animal and concerned only minor bacterial taxa such as bacteroidales S24_7 group family and the genus Syntrophococcus, Lachnosporacaeae NC2004 group, Ruminiclostridium and Eubacterium cellulosolvens group.