A range of granular sludges was taken from industrial anaerobic sludge blanket reactors treating a wide variety of wastewaters and a comparison was made between their surface and internal structures. Scanning electron microscopy showed a similar surface structure of heterogeneous bacteria, regardless of the type of wastewater being treated. However, a study of the internal structure, using sequential staining of ultra-thin sections, showed the complexity of granular sludges. The majority of the internally-located bacteria had a morphology similar to Methanothrix. Much of the area was occupied by Gram-negative organisms and the area which stained positive for protein was found to increase nearer the centre of the granules. This was accompanied by a decrease in the carbohydrate-positive areas. Positive areas for lipid were widespread throughout the granules. Changes in the internal structure were observed when the type of wastewater treated by the granules was changed and a comparison between sludges treating the same type of wastewater showed that other factors, such as organic loading rate, pH and chemical oxygen demand may also affect the structure of the granules.