Abstract The relationships between bacterial concentration, bacterial production, and cell-specific activity of both free and attached bacteria and environmental factors such as suspended solids, nutrients, and temperature were examined in four lakes, two in New Zealand and two in Switzerland. Estimates of cell-specific production were obtained by microautoradiographic counts of [3H]thymidine-labeled cells. Bacteria attached to particles accounted for only 1.3 to 11.6% of the total bacterial abundance, but showed overall 20-fold higher specific growth rates and were relatively more active than their free counterparts. On average, 80 to 100% of epibacteria were attached to organic particles. The abundance and production of free and attached bacteria were positively correlated; however, relationships between these fractions and some environmental variables differed. Cell-specific activities of active bacteria were not equivalent to mean cellular activities of the entire bacterial community and differed in their relationship to trophic state. [3H]Thymidine-positive bacteria were more tightly linked to chlorophyll a than were total bacteria. Our findings indicate that production by attached bacteria, fueled by phytoplankton carbon, supplies "new" free bacteria to the bacterial community. Our results support the idea that particulate organic matter acts as a source of dissolved nutrients to free bacteria. Bottom-up control of bacterial biomass, as shown by regressions of biomass vs production, appeared to be stronger in two ultraoligotrophic lakes than in two more eutrophic ones.