In healthy pigs aged one to 35 days significant populations of microorganisms (5.5 to 6.9 log10 viable count cm-2 of the tissue) were found, by strict anaerobic techniques, to adhere to the pars oesophagea. The genera of anaerobes which were isolated included Actinomyces, Bifidobacterium, Clostridium, Lactobacillus, Peptostreptococcus, Streptococcus and Veillonella and facultative anaerobes included Escherichia, Klebsiella, Staphylococcus, Streptococcus and yeasts. The microbial population adhering to the pars oesophagea varied little from birth till after weaning and Lactobacillus, Clostridium and Eubacterium predominated. There were generally small numbers of facultatively anaerobic microorganisms in the pars oesophagea of healthy sucking or weaned pigs but large numbers of anaerobes, particularly Lactobacillus. In the pars oesophagea of sucking and weaned pigs with diarrhoea, large numbers of facultative anaerobes were frequent but Lactobacillus were also present in large numbers.