Unsaturated fats when heated undergo peroxidation, leading to the formation of oxidation products. For example, that of cis-9,cis-12-C18:2 forms majorly 13OOH cis-9,trans-11-C18:2 (13HPOD), hexanal and trans-2,trans-4-decadienal (T2T4D). In ruminants, some studies using heated seeds and oils reported an effect of such heated fats on ruminal metabolism of fatty acids, which could affect milk and meat fat qualities for consumers. One possible explanation is a modification of ruminal bacterial community, as peroxides and aldehydes are known to have antimicrobial effects. In order to investigate the effects of these 3 oxidative products on ruminal microbiota, a long term in vitro study was conducted. Rumen fluid was incubated with one of the three oxidation products for 54h or 102h. Blanks without oxidative products served as fermentations and bacterial population control for both incubation durations. Bacterial density (qPCR), diversity (Simpson index) and community (CE-SSCP) were estimated in incubated ruminal cultures. Density and diversity indexes of bacterial community were analyzed using the General Linear Model. To compare the structure of the bacterial communities, the pairwise Euclidean distances of the CE-SSCP profiles were used. Global ANOSIM was performed to test the fixed effects of oxidation products. The ANOSIM-R value indicates the extent to which the groups differed and is between 0 (no separation) and 1 (well separated groups). When a fixed effect was significant compared to blank, an iterative Mann-Whitney test was carried out on the 1700 scans of the CE-SSCP profiles to estimate the percentage of population that differed between blank and the treatment. Bacterial density was not modified by treatments: 1.1010 DNA copies / ml of cultures, on average. Hexanal and 13HPOD had no effect on ruminal bacterial diversity and community, when T2T4D modified them. Bacterial diversity in T2T4D cultures was 7.6 vs. 8.0 for blanks. Bacterial community was strongly affected by T2T4D and this increased with incubation duration: differences were of 21% (R-ANOSIM = 0.46) after 54h and 49% (R-ANOSIM = 0.92) after 102h of incubation between blanks and T2T4D (P < 0.05). Ruminal microbiota was strongly modified by T2T4D. Bacterial species sensitive to T2T4D need to be identified and effects on other enteric microbiota verified.