Repeated flow cytometric analyses of population shifts within a bacterial consortium growing under sulfate-reducing conditions revealed unexpected and quick changes in forward scatter vs. side scatter dot plot patterns within less than 30 min of exposure to air. These pattern changes proceeded differently when cells were cultivated either at 30 DegC or 12 DegC. It was found that prominent sub-distributions, clustering profoundly in size and no. of events with time, represented sulfur particles and not microbial cells. Therefore, any interpretation of flow cytometric analyses of live bacteria grown under sulfate-reducing conditions requires specific caution since sulfur particles might cause errors in data interpretation. To prevent artifacts, distinct treatments of the sample are recommended.