Nylon bags of different mesh size (5-53 μm), containing ground lucerne, were incubated in the rumen of sheep fed lucerne hay. Counts of total culturable, proteolytic and cellulolytic bacteria were performed on the bag contents after 16 h of incubation and also on a ruminal ingesta sample in an anaerobic cabinet using the plating technique. Microscopic counts of ciliate protozoa were also made. Total culturable counts in the 5 and 10 μm bags were <10% and in the 53 μm bag only 60% of the values in ruminal ingesta. However, counts of the ciliate protozoa were higher in the 30 and 53 μm bags than in the ruminal ingesta probably due to their greater ability to move into the bags with larger mesh size. The results show that the microbiology inside the bag differs from that of the surrounding ingesta and care should be taken in interpreting results on feed evaluation and degradation obtained using this technique.