Gram negative budding bacteria with a characteristic morphology have been observed in the caeca of chickens, turkeys, ducks and guinea fowl at levels between 0–4 and 24% of the total flora. Using strict anaerobic techniques a detailed study has been made of the organisms isolated from the caeca of chickens aged between 15 and 44 d. The majority of isolates were identified asGemmiger formicilis, the strains being divided into twomajor types according to the relative amounts of butyric acid and lactic acid produced from glucose. A second group of isolates could not be related to any known species.Amongst properties of ecological interest studied were limiting temperatures for growth, ammonia utilization, ability to utilize various carbon sources and survival in environments outside the intestine.Attempts to produce a selective medium for the isolation of these organisms when present at less than 2% of the total flora were unsuccessfu