The aim of this study was to examine, over a period of 1 year, interindividual variations in the most prominent and representative of the cultivatable microbial populations in the feces of eight healthy Spanish persons. A number of biochemical variables (enzyme activities and ammonium and short-chain fatty acid [SCFA] concentrations) thought to be influenced by the GIT microbiota were also analyzed. Total cultivatable microbial counts ranged from 10(10) to 10(11) cfu/g of feces. The largest populations were obligate anaerobes belonging to the Clostridium clusters, followed by species of bifidobacteria and bacteroides. Coliforms and lactobacilli were found at a more intermediate level (10(5)-10(9) cfu/g). The predominant anaerobe populations remained quite constant over time, but all other microbial groups showed significant interindividual differences. Enzyme profiles were individual-dependent, but within subjects, moderate to high intersample variations over time were recorded for some activities. Fecal ammonium concentration was the most unpredictable variable; this fluctuated widely between individuals and samples. Acetic acid was the most abundant SCFA in the feces, followed by butyric and propionic acids. SCFA concentrations also varied according to the individual; some subjects showed specific profiles in terms of SCFA composition or concentration. The fecal microbial and biochemical parameters studied seemed to be individual-dependent. Most variables were rather stable over time, while others (e.g., ammonium concentration) varied widely.