The nutritional benefits of lactic acid bacteria in fermented dairy products have been well documented, especially in terms of weight gain and feed efficiency, but not in terms of small intestine adaptation. The effects of a diet supplemented (30% wt/wt) with milk fermented either by Lactobacillus casei DN-114 001 or yoghurt for 3 or 15 days were investigated in the small intestine of mice by morphometry, kinetic analysis and determination of brush-border enzyme activities. Results were compared with those obtained with standard or milk isocaloric diets. Cell proliferation and villous area were significantly increased in the proximal intestine of mice fed the fermented-milk-supplemented diets for 3 days and were associated with hypertrophy and hyperplasia of Paneth and goblet cells. Lactase-specific activity was increased by fermented-milk diets at days 3 and 15, whereas there was no variation in maltase-specific activity. Alkaline phosphatase-specific activity was increased after 3 days of the three tested diets in the whole intestine, and after 15 days in the proximal intestine. Aminopeptidase activity was increased in the distal part of the intestine after 3 days of the 3 diets. Our findings suggest that diets supplemented with fermented milks have a positive effect on the trophicity of the mucosa in the small intestine of mice.