Nine of 14 cases of giardiasis and severe malabsorption were found to have numerous bacteria adjacent to the mucosa and within luminal fluid samples from the upper jejunum. Three species of enterobacteria (Klebsiella pneumoniae, Enterobacter cloacae and E. hafniae) were cultured from eight patients and from only one were Bacteroides isolated. Enterobacteria were not cultured from seven of eight patients who had giardiasis but only mild malabsorption (of xylose only) nor from seven patients without malabsorption. Intestinal colonization by enterobacteria may make an important contribution to the development of malabsorption in patients with giardiasis.