An in vivo study was carried on rats to see the influence of arsenic on aerobic gut flora. A significant inhibition of gut flora was observed after 2 weeks of administration of arsenic (1 mg/L) ad libitum with a decrease in stool arsenic level and increase in liver arsenic level. However, this inhibitory effect of arsenic on gut flora was not observed in presence of vitamin E (1 mg/day) or selenium (0.4 µg/day). Pretreatment with streptomycin (500 mg twice daily) showed similar results. Rats that received folic acid (200 µg/day) showed inhibition of gut floral count but there were decreased liver arsenic level.