ABSTRACT. Little IS known about the relative significance of heterotrophic bacteria in open-ocean oligotrophic environments. The pelagic waters of the Levantine Basln of the eastern Mediterranean Sea are among the most oligotrophic on record. We surveyed the spatial distribution of bacterial abundance, biomass and production along 2 transects of the pelagic waters of the southern Levantine Basin to assess which changes in these parameters may occur in association with varylng physlcal structure and c.hlorophyl1 concentrations, to calculate the relative b~o m a s s contl-ibutions of bacter~a and phytoplankton, and to estimate the magnitude of carbon flux from phytoplankton to bacteria. Chlorophyll had an average concentration of 134 * 85.4 n g 1-' and was relatively uniform throughout the upper 200 m Bacterial numbers ranged from 0.40 to 3 90 X 10H cells 1.' and were generally highest above l10 m. Cocci cells comprised 87";1 of the population with an average volume of 0.049 pm" Bacterlal numbers and biomass were notably high in the Ierapetra Eddy and Mersa Matruh Gyre. Although bacterial numbers and chlorophyll concentrations were not generally correlated, the mean bacterial number was accurately predicted from a regression equation uslng chlorophyll. Over the upper 200 m , bacterial b~omass (x= 603 mgC m-?) was on average about 50% of phytoplankton biomass (2 = 1235 mgC m l ) , which is contrary to other published studies reporting bacterial biomass equalled or exceeded algal biomass in oligotrophic marine waters. Bacterial production ranqed from 0 to 3.91 pm01 TdR 1-'