The objective of this study was to investigate the effects of donor’s cow diet (hay or maize silage plus concentrates) on ruminal bacteria count, flora diversity and fatty acids profile (FA) of ruminal fluid and in vitro biohydrogenation (BH) of C18:2. Two dry cows fitted with a ruminal canula were used in a 2x2 design. Each period included three weeks of diet adaptation and two weeks of sampling. The cows were fed twice daily either a diet (H) composed of grass (38%) and alfalfa hay (62%) or an acidogenic diet (A) composed of maize silage (38%), wheat (57%) and soybean (5%) meal. Ruminal fluid was sampled and centrifugated (150g, 5min., 39°C). The ruminal fluid (80mL) was mixed with 80mL of buffer, a fermentative substrate and grape seed oil as source of C18:2 before being incubated during 6 hours at 39°C in anaerobic and dark conditions. Biodiversity was estimated by the Simpson index modified by Haegeman et al.1 after SSCP analysis, and FA were analysed by GLC. Bacteria counting was realised according to Oblinger and Koburger2 (1975). Total and cellulolytic bacteria contents were higher in inoculum A than in inoculum H (9.3.109 vs. 2.4.108/mL for total bacteria and 2.4.108 vs. 1.6.107/mL for cellulolytic bacteria). No difference in the biodiversity of the inoculums was noticed according to the cow or the diet, but diversity during period 1 tended to differ (P=0.09) from period 2, suggesting a time variation of flora biodiversity. Before incubation, the ruminal fluid from the cow receiving diet A contained significantly (P<0.01) more C18:2, trans-10 and trans-11 C18:1, and odd-chain FA than inoculum from the cow receiving diet H. After incubation, inoculum A resulted in a significantly (P<0.01) greater BH of C18:2 than inoculum H, and produced more trans-10C18:1, trans-11C18:1 and odd-chain FA (P<0.01) . Trans-10 and odd-chain FA are known to be increased by a high concentrate diet, which explains that inoculum A was richer in these FA than inoculum H. The ruminal flora selected in vivo by diet A continued the production of these FA in vitro. The greater content of trans-11C18:1 and of C18:2 in the inoculum A could be explained by the greater content in C18:2 of the diet A. During incubation with added C18:2, inoculum A continued to produce more trans-11 along with a higher C18:2 BH than inoculum H, which could be due to the higher concentration of cellulolytic bacteria in the inoculum A.