ABSTRACT: The metabolic activity associated with seston in Lake Kinneret, Israel, was estimated as specific (per unit dry weight, DW, or particulate organic carbon, POC) respiration during experimental studies in 1995. Some determinations of oxygen demand by seston and lake snow aggregates were also carried out in Lake Constance, Germany. About 86 and 84% of the variation in total community respiration in the epilimnetic water of Lake Kinneret could be explained by the variability of seston dry weight and the POC content, respectively. The mean specific respiration of seston was 0.10 ± 0.006 mg O₂ mgDW^-1 d^-1 (or 0.07 ± 0.005 mg C mgPOC^-1 d^-1). These values characterize the seston as a metabolically active compartment of the lake ecosystem with a mean turnover time of about 14 d. Experiments with aggregated seston of different origins indicated a tendency toward decreasing specific metabolic activities with increasing aggregate size. The mean specific biomass production of seston-associated bacteria was 0.014 ± 0.001 mg C mgPOC^-1 d^-1. The theoretical limit of the growth efficiency of seston-associated bacteria was also estimated using the parameters of specific respiration and specific bacterial production.