A basic mechanism implicated in the human health-promoting properties of probiotics is their ability to maintain the homeostasis of the intestinal microbiota. This study evaluates how the ingestion of different amounts of the probiotic Lactobacillus johnsonii La1 (La1) influences the main bacterial populations of the fecal microbiota. Eight asymptomatic volunteers participated in the study. After a basal period, they ingested 100 mL of a product containing 10(8) La1/mL during the first week, 200 mL during the second week and 500 ml during the third week. Fecal samples were obtained at the end of each period and during the 2 weeks post-ingestion. Lactobacilli were determined by culture on MRS agar and La1 colonies were confirmed by ERIC-PCR. The main populations of fecal bacteria were identified by fluorescent probes and flow cytometry. At baseline, 18.3% of the total fluorescent bacteria were F. praunitzii, 13.2% Bacteroides, 2.05% Bifidobacterium and 0.95% Lactobacillus. Fecal excretion of La1 increased during the ingestion period but it was cleared from the stools of the volunteers 2 weeks later. La1 intake increased the populations of Lactobacillus (p=0.056) and Bifidobacterium (p=0.067), which are considered as beneficial for the host, while it decreased those of F. prausnitzii (p=0.005) a potentially pathogenic microorganism. These bacterial populations returned to their baseline levels during the post-ingestion period. The regular intake of a La1-containing product beneficially affects the homeostasis of the human fecal microbiota probably contributing to the health-promoting effects of this probiotic