This study aimed at comparing various diets predicted to induce different stimulations of the cecal microbial activity of the young rabbit fed ad libitum from 16 to 70 d of age: i) a diet enriched with rapidly fermentable fiber expected to stimulate the cecal microbial activity (RFF group); ii) a control diet with a standard composition (C group); iii) and the same control diet with tiamulin and apramycin antibiotics, expected to inhibit the microbial activity (C+AB group). A total of 398 rabbits were used from 42 litters and weaned at 28 d of age. An in vivo digestibility trial was performed on 36 rabbits of 42 to 46 d of age housed in individual metabolic cages. The feed intake and growth rates were lower in the RFF group compared with the C+AB group (-15% in ADFI and -11% in ADG, P < 0.001), with a lower weight of -183 g at 70 d (P < 0.001). No significant difference was found on ADG and final BW between the RFF and the C groups, but the RFF diet allowed a better G: F ratio at postweaning (P < 0.01). The digestion of soluble fiber (total dietary fiber minus NDF) was greater for the RFF group. The C+AB diet had a positive effect on the postweaning morbidity rate (P < 0.05) but did not affect the mortality rate and the health risk index (morbidity and mortality). Conversely, the RFF diet appeared to reduce the mortality rate compared with the C+AB diet, especially before 41 d of age. Concerning the cecal microbial activity, a supply of RFF in the diet increased the cecal VFA concentrations (+28% vs. C+AB and +22% vs. C, P < 0.001) and lowered the pH. The VFA pattern was affected at 45 and 60 d, with a dominance of acetate in the RFF group (+4% vs. C+AB and C groups, P < 0.001) instead of butyrate in the C+AB and C groups (-3.6% and -5% vs. C+AB and C, respectively, P < 0.001). Antibiotics addition (C+AB group) reduced the VFA concentration, but only after weaning (-25% at 45d of age) without changing the fermentation pattern. In conclusion, early intake of RFF in young rabbits stimulated the cecal microbial activity, and reduced the voluntary feed intake, leading to a reduced G: F ratio.