A fluoreseently-labelled r-RNA targeted oligonucleotide probe specific for members of the genus Amaricoccus, which includes one group of the Gram-negative G-Bacteria seen in activated sludge systems, is described. These organisms, previously 'identified' on their distinctive morphology of cocci in tetrads, have been associated with poor performance of biological nutrient removal (EBNR) plants, by out-competing the polyphosphate accumulating bacteria, Methods of sample preparation for probing activated sludge are detailed, and preliminary surveys of 46 plants, using this probe, show that G-Bacteria belonging to the genus Amaricoccus are seen not only in large numbers in EBNR systems but also in conventional plants. The presence of single cells of this organism was common, emphasizing the dangers of relying morphology and cell arrangement to identify these bacteria.