A study of the distribution of bacteria in relation to particle concentration and type was conducted over a spring-neap tidal cycle in the Tamar Estuary, southwest England. Three groups of bacteria were recognized: free-living; those attached to permanently suspended particles; and those attached to particles which undergo tidally controlled resuspension and sedimentation. The total activity and the activity of all three groups of bacteria increase in the turbidity maximum region. The bacteria associated with the permanently suspended particles, which have a larger mean size and organic carbon content than those in the resuspended sediments, contribute the major part of this increased activity. This is a significant finding as it had been previously thought that the increase in bacterial activity at the turbidity maximum was due to bacteria attached to resuspended sediments. However, resuspension still plays an important role because the increase in bacterial activity is consistently coincident with the turbidity maximum.