Weaning of the pig is generally regarded as a stressful event which could lead to clinical implications because of the changes in the intestinal ecosystem. The functional properties of microbiota inhabiting the pig's small intestine (SI), including lactobacilli which are assumed to exert health-promoting properties, are yet poorly described. Thus, we determined the ecophysiology of bacterial groups and within genus Lactobacillus in the SI of weaning piglets and the impact of dietary changes. The SI contents of 20 piglets, 4 killed at weaning (only sow milk and no creep feed) and 4 killed at 1, 2, 5, and 11 days post weaning (pw; cereal-based diet) were examined for bacterial cell count and bacterial metabolites by fluorescence in situ hybridization (FISH). Lactobacilli were the predominant group in the SI except at 1 day pw because of a marked reduction in their number. On day 11 pw, bifidobacteria and E. coli were not detected, and Enterobacteriaceae and members of the Clostridium coccoides/Eubacterium rectale cluster were only found occasionally. L. sobrius/L. amylovorus became dominant species whereas the abundance of L. salivarius and L. gasseri/johnsonii declined. Concentration of lactic acid increased pw whereas pH, volatile fatty acids, and ammonia decreased. Carbohydrate utilization of 76 Lactobacillus spp. isolates was studied revealing a shift from lactose and galactose to starch, cellobiose, and xylose, suggesting that the bacteria colonizing the SI of piglets adapt to the newly introduced nutrients during the early weaning period. Identification of isolates based on partial 16S rRNA gene sequence data and comparison with fermentation data furthermore suggested adaptation processes below the species level. The results of our study will help to understand intestinal bacterial ecophysiology and to develop nutritional regimes to prevent or counteract complications during the weaning transition.