Glass fiber filters are widely used to concentrate and collect a variety of particles suspended in seawater. The filters are particularly useful for chemical analyses of the filter-retained particles, because of the physico-chemical stabihty of the matenal the filters are made of. Despite this usefulness, very small particles, e.g. many planktonic bacteria, are known to pass the glass fiber filters, and little information is currently available on the fraction passing the filters. In this study, natural bacterioplankton cells were filtered through Whatman GF/F filters, and apparent cell size-frequency distributions were obtained by epifluorescence photomicrography before and after the filtration. Seawater samples were from a northwest Atlantic coast and an Antarctic sea. Filtrate contained 35 to 43 % of the total bacterial cell count, equivalent to 22 to 38 % of the total bacterial biomass. Size-frequency distributions showed that cells larger than 0.8 pm diameter (volume-equivalent spherical diameter, VESD) did not pass the filters, however the filters retained substantial numbers of small (VESD <0.8 pm) bacteria. In comparisons of the different types of glass fiber filters, retention efficiency (fraction of the cells retained by a filter) ranged from ca 13 to 51 %, i.e. 49 to 87 % of the natural bacterioplankton cells passed through the tested filters. Interpretations of data obtained via filtrations through glass fiber filters must properly consider the fraction not retained by the filters.