Ochre deposits from sites in England and Wales varied widely in their composition, their appearance, their rate of formation and where they occurred. It was demonstrated that chemical oxidation could account for the precipitation of iron from drainage water. In sterile samples of drainage water from ochreous sites 80% of the total iron in solution was oxidized within 48 hours. It was also shown that autocatalysis of ferrous iron oxidation by ferric precipitates could occur in drainage water. Filamentous bacteria were observed by light microscopy in most samples and were assigned to the genus Leptothrix. Gallionella 'app were observed in some deposits but their distribution was not widespread. Shaerotilus slop_ were not found by light microscopy or isolated in artificial media. The chemolithotroph T.ferrooxidans was found in acidic ochre samples (pH44.0) and in samples from drainage water of near neutral pH suggesting that it can survive in microenvironments of low pH, contributing to ochre formation over a wide range of drainage water pH. Heterctrophic bacteria capable of growing in artificial media of low pH were isolated, primarily from acidic samples, and the results suggested that they were polysacchuride producing. Complex degrading heterotrophic bacteria were also isolated from ochre using a ferric ammonium citrate medium. Some deposits, on the basis of chemistry and microbiology, could be described as either pyritic or filamentous ochre. However, the majority of samples fell between these extremes and had various combinations of filamentous bacteria, Thiobacilli and heterotrophic organisms. 7 A marine antifouling paint containing copper was used to control the growth of sheathed filamentous bacteria in drainage water and other heterotrophic bacteria in artificial media. Since ochre results from the interaction of many factors, chemical and microbial, the use of copper applied as an antifouling paint or incorporated into drainage pipes, must be proved effective and economic in relation to ochre prevention in agricultural drainage systems.