The spatial location of microorganisms and their activity within the soil matrix have major impacts on biological processes such as nutrient cycling. However, characterizing the biophysical interface in soils is hampered by a lack of techniques at relevant scales. A novel method for studying the distribution of microorganisms that have incorporated isotopically labelled substrate ('active' microorganisms) in relation to the soil microbial habitat is provided by nano-scale secondary ion mass spectrometry (NanoSIMS). Pseudomonas fluorescens are ubiquitous in soil and were therefore used as a model for 'active' microorganisms in soil. Batch cultures (NCTC 10038) were grown in a minimal salt medium containing N-15-ammoniurn sulphate (N-15/14 ratio of 1.174), added to quartz-based white sand or soil (coarse textured sand), embedded in Araldite 502 resin and sectioned for NanoSIMS analysis. The N-15-enriched P. fluorescens could be identified within the soil structure, demonstrating that the NanoSIMS technique enables the study of spatial location of microbial activity in relation to the heterogeneous soil matrix. This technique is complementary to the existing techniques of digital imaging analysis of soil thin sections and scanning electron microscopy. Together with advanced computer-aided tomography of soils and mathematical modelling of soil heterogeneity, NanoSIMS may be a powerful tool for studying physical and biological interactions, thereby furthering our understanding of the biophysical interface in soils. Copyright (c) 2006 John Wiley & Sons, Ltd.