Biodegradation of drilling fluid (water based and oil based drilling fuild) used in the upstream sector of the Nigeria petroleum industry was investigated in marine water environment. Drilling fluid were obtainedfrom Izombe well drilling site Owerri and marine water sample from Bonny River, Nigeria. The methodology of ultimate biodegradability estimated from the ratio of Biochemical Oxygen Demand to Chemical Oxygen Demand was used to determine biodegradability. Evaluation of percentage (%) ultimate biodegradation at day 20; showed that water based drilling fluid had 59.5% to be more biodegradable than Oil based drilling fluid with 54.2%. Drilling fluid utilizing bacteria genera isolated were: Pseudomonas, Bacillus, Micrococcus and Enterobacter, with Pseudomonas having the highest frequency of 35.7%, followed by Bacillus with the frequency of 30.7%, Micrococcus had 15.4% and Enterobacter 15.4%.Fungi genera isolated include Aspergillus, Penicillium, Rhizopus Mucor. Conclusively, the study showed that water based drilling fluid is more biodegradable in marine water environment than oil based drilling fluid. Therefore it is recommended that since most oil well drilling activities in Nigeria are carried out in marine environments, water based drilling fluid should be the best option due to its high degradability rate.