Over the last four years, there has been an increasing occurrence of biofouling in pencross bent grass greens on sand based media. The major cause is the occurrence of a biologically‐synthesised horizontally‐stratified black plug layer (BPL). To detect these layers, computer assisted tomography (CAT scan) was successfully applied to an infested core. A much clearer definition of the layers was obtained than by visual observation. Bacteriological changes were also observed, with the iron‐related bacteria appearing to be a more significant component in the BPL. The iron‐related bacteria may have seen subject to a lower level of stress than isolates above and below the layer.