This paper presents the results concerning total number of cells capable of growth and multiplication (direct viable count) in marine microcosms supplemented with gasoline and gasoline-enriched marine populations. The direct viable count is further discussed in correlation with the BOD5 of these samples in the attempt to take into account the intensity of biological oxygen consumption together with the number of active growing microorganisms. The linear correlation between the number of cells capable of growth and multiplication (direct viable count) and BOD5 in the five microcosms is good (r= 0.96). Up to our best knowledge this is the first attempt to simultaneously measure these two parameters in marine microcosms polluted with hydrocarbons (gasoline).