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The chemical/physical environment of groundwater may contribute to the existence of a subpopulation of small-sized bacteria (filterable bacteria) that fails to be trapped on conventional 0·45 μm-pore-size membrane filters during routine bacteriological water quality analyses. Efforts were directed to determining an efficient recovery method for detection of such cells.Individual groundwater supplies in a rural setting were examined by a double membrane filtration procedure to determine the presence of heterotrophic plate count (HPC) bacteria capable of escaping detection on conventional pore size (0·45 μm) membrane filters but retained on 0·22 μm-pore-size filters. Since optimum cultural conditions for recovery of filterable bacteria are not well defined, initial efforts focused on evaluation of various media (R2A, m-HPC and NWRI) and incubation temperatures (15, 20, 28 and 35°C) for specific recovery of filterable bacteria. Maximum recovery of small-sized HPC bacteria occurred on low-nutrient concentration R2A agar incubated for 7 d at 28°C. Similarly, identical cultural conditions gave enhanced detection of the general HPC population on 0·45 μm-pore-size filters. A 17-month survey of 10 well water supplies conducted with the cultural conditions described above resulted in detection of filterable bacteria (ranging in density from 9 to 175 cfu ml−1) in six of the groundwater sources. The proportion of filterable bacteria in any single sample never exceeded 10% of the total HPC population. A majority of the colonies appearing on the 0·22 μm membrane filters was pigmented (50–90%), whereas the proportion of colonies demonstrating pigmentation on the larger porosity filters failed to exceed 50% for any of the samples (19–49%).A reliable recovery method was developed for the detection of filterable bacteria from groundwater. During a subsequent survey study using this procedure, filterable bacteria were detected in a majority of the groundwater supplies examined; however, the density of filterable bacteria in any single sample never exceeded 10% of the total HPC population. Identification of randomly selected isolates obtained on the 0·22 μm filters indicated that some of these filterable bacteria have been implicated as opportunistic pathogens.We have determined the presence of small-sized HPC bacteria in ground water that may go undetected when using standard porosity membrane filters for water quality analyses. Further study is needed to assess the significance and possible health risk associated with presence of filterable bacteria in drinking water supplies from groundwater sources.