The objective of this study was to assess the effects of commercially available feed additives on the gut microflora of finishing pigs. Pigs received either a barley-based control diet, or an experimental diet supplemented with mannanoligosaccharide (BioMOSì), fumaric acid, or a commercially available acid/salt mixture (Bact-A-Cidì) for four weeks prior to slaughter. Dietary supplementation with fumaric acid (20 g/kg) resulted in the greatest effects on gut microflora composition. Following 28 days of treatment, faecal coliforms and lactobacilli numbers were reduced in the fumaric acid-fed animals (P<0.05). In addition, there was a ten-fold reduction in lactobacilli in the caecum and colon due to fumaric acid treatment (P<0.05). The data indicate that supplementation with fumaric acid caused a desirable change in coliform numbers. However, given that Lactobacillus are considered beneficial microorganisms in the mammalian intestine, the reduction in lactobacilli counts as a result of fumaric acid supplementation warrants further investigation.