This preliminary work shows ESR (Electron Spin Resonance) can be used to detect biofilms, particularly from Fe-metabolising bacteria. A film was detected by ESR as early as 1 day, hence possibly more sensitively than by fluorescent methods. Films can probably be detected as early as one hour. Spectra contain a very broad peak at g=2.13, probably due to ferrihydrite. Results of field experiments from streams and ponds in New Zealand and Japan, particularly the Minoh River, showed a general increase of ferrihydrite with time. Loss by exfoliation was later than 20 days. The rate of accumulation was faster in a nutrient-rich stagnant pond. Hematite (g=4.3) was often observed, magnetite (g=9) once, and usually small amounts of a common bacterial decay product. The latter was detected for at least 18 months film storage. ESR is a particularly good tool for observing the growth of oxic biofilms containing Fe-metabolising bacteria, and should be just as sensitive for observing Mn-metabolising bacteria in reducing conditions.