Biotechnology improved water flood performance during a two-year test in Victor Claro reservoir layers C2-D2, La Ventana field, northern Cuyo Basin, Mendoza Province, Argentina (Figure 1, 2 and 3). Paraffinic oil has been produced by water flooding since 1989. The objective of this project was to determine the technical and economical feasibility of Microbial Enhanced Oil Recovery (MEOR) methods for field-wide application in mature water floods. This paper explains the continuation of a pilot study of controlled microbial colonization in producer wells begun in La Ventana field in 1995. Fluid property alterations were evaluated using a complete set of rheology parameters, specific geochemical fingerprints and bioassays on more than 82 samples from La Ventana and neighboring fields. The oil proved to be highly bio-treatable and formation water to be compatible with MEOR requirements. Reservoir conditions, including rock properties, pressure and temperature were also favorable. Microbial Inoculation Program started December 1998 and was implemented by systematic treatment of injected water using hydrocarbon degrading anaerobic-facultative microorganisms on a non-regular injection pattern having direct influence on four neighboring producers (Figure 4). A complementary tracer field test using Tritium permitted validation of inter-well connectivity and allocation factors initially inferred by the use of streamline simulation. Pre-MEOR and Post-MEOR fractional flow field performance in producers was also correlated with theoretical predictions using a proprietary 2D, vertically integrated streamline simulator. The tracer and simulations contributed to better understanding MEOR mechanisms and to better designs for bio-treatments. Original oil production decline rates improved. Increments in oil cut were measured using conventional methods for 24 months after MEOR program started (Figure 19). Two combined mechanisms help explain the change: 1. Further depletion of residual oil saturation by in-situ production of metabolites with low interfacial tension and 2. Improved oil mobility from short-chain solvents originated by microbial alteration of heavy paraffinic ends. Further evaluations will cover extension of pilot period and economic feasibility analysis for large-scale MEOR implementation in other zones under conventional flooding.