Reports on the initial colonization by bacteria and the development of slime films on titanium, copper, 90/10 copper-nickel, and aluminum brass surfaces exposed in the sea and in the Thames Estuary. Using transmission electron microscopy it has been shown that a primary film of bacteria develops much quicker on titanium surfaces (4 weeks) than on copper (22 weeks) and copper alloys (19-22 weeks). Sections taken from the surface of these alloys show that bacteria were intimately associated with the corrosion products. This is particularly evident on copper-nickel surfaces where bacteria are initially found between alternate layers of corrosion products. These results are discussed with respect to the fouling process, the formation of corrosion layers, and the use of these alloys in the marine environment.