The use of sulphate-reducing bacteria (SRB) in the passive treatment of acid mine drainage (AMD) requires the presence of a sediment, such as gravel bed, in which the bacteria can establish microenvironments conducive to their survival in the presence of oxygen and acidity. The characterization of relationships between the physical environment and SRB activity would be useful in the sizing of passive treatment systems using SRB. The effects of physical parameters of a gravel bed (i.e. void volume, total surface area of gravel, type of gravel) on SRB activity while in the presence of varying loads of fresh AMD was investigated. SRB activity was expressed as the proportion of sulphate removed after 7 days. The study was accomplished in a series of column reactors containing beds comprising different types and size I:lasses of gravel. SRB were established in the beds and metal and sulphate removal were monitored. Quantifiable relationships were found to exist between SRB activity and the physical parameters of the bed.