The study of electron transport system (ETS) in marine zooplankton provides potential respiration rates from measurements of the maximum activity of the enzymes involved in the respiratory electron transport chain. From these measurement we calculate the metabolic carbon demand to elucidate which group of zooplankton are most influential in the epipelagic zone carbon cycle. Here we present observations from a study of the zooplankton community in the upper 200 meters of the water column over the Balearic sub-basin and Algerian sub-basin, two areas in the oligotrophic Mediterranean with different oceanographic conditions. We compared the biomass, ETS activity and specific ETS activity of different fractions of size (53-200, 200-500, >500μm) in both areas. In the allometric exponential relationship between biomass and respiration, the coeficient b was less than 0.75 (Kleiber’s law). This supports the hypothesis of Gómez et al. (2008) that deviations from Kleiber’s law are influenced by nutritional state. When b 0.75 they are relatively well-fed. In both regions of this study the largest contribution to the ETS activity and to the potential respiration is by the larger sizes, while for the specific respiration (per unit biomass), smaller fractions are more important. This indicates that the small size fractions are, metabolically, more active. Both biomass and ETS activity increased in the region the Algerian sub-basin and for both regions the biomass and the ETS activity is greater in shallower water (200 m vs. 900m). 62 54 1,231 2,508 Q1 Q1 SCIE