We measured bacterial growth rates by labelled thymidine incorporation, grazing loss rates with fluorescent latex particles, and bacterial cell size and abundance within narrow size-fractions of freshwater bacterioplankton. Contrary to the predictions of standard allometric relationships, the smallest bacteria showed the lowest incorporation rate per cell and per unit DNA content. Cells trapped by 1- and 5-mum filters grew the fastest and were responsible for 86% of detected thymidine biosynthesis. Grazing studies in six lakes showed that grazing pressure from flagellated protozoans and mixotrophic algae on large bacterial cells was probably 2-40 times more intense than that on the smallest cells. We suggest that reduced grazing pressure on the smallest bacteria allows them to dominate numerically, despite their slower growth.