The presence of 'ultramicrocells' in natural mineral water, capable of passing through a 0.2 μm filter, has been demonstrated. Filters allowing the greatest proportion of viable (culturable) cells to pass ranked in the order, 0.4 μm polycarbonate (5.02%) > 0.2 μm polycarbonate (0.02%) ≥ 0.45 μm cellulose nitrate (0.02%) > 0.2 μm cellulose acetate (< 0.002%). Following incubation for 4d at 22 °C, viable counts in filtered mineral water increased from < 2-8.7 × 102 cfu ml−1-2.8 × 104-1.9 × 106 cfu ml−1. Successive filtration/incubation cycles of mineral water increased the proportion of cells passing through a 0.2 μm cellulose acetate filter from < 0.003% to 0.11% and 0.69%, suggesting selection for 'ultramicrocells'. Cells isolated from this process and grown on liquid R2A medium were thin, Gram-negative rods, of 0.15-0.40 μm wide and 0.50-6.20 μm long. Membrane filtration techniques used for pathogen detection in mineral waters will not retain all the cells present. If pathogens are able to form ultramicrocells, these may go undetected.