Abstract More than 300 oil producing wells have been stimulated with microorganisms in Lake Maracaibo, western Venezuela, since 1996. Most of these wells are managed by Tía Juana E.U. (Exploitation Unit), PDVSA E&P, Western Area, and belong to reservoir LGINF-05. This reservoir is constituted by four different unconsolidated Miocene sandy intervals, at an average depth of 3,800 feet, average porosity 30%, average permeability to oil of 600 mD and average oil gravity of 25° API. The wells were injected at the platform using a self-propelled barge, equipped with a triplex pump, at an average rate of 3 bls/min. and were shut-in for 7 days. Fifty (50) wells stimulated in 1997 were monitored for at least 6 months after the injection. Production data is reported and a correlation was made with different geological and reservoir data, to determine the parameters that mostly influenced the effectiveness of the responce. The results showed a 78% success, with production increments ranging from 30 additional BOPD, to 400 additional BOPD, with an average of 65 additional barrels of oil produced per day. Correlation results showed a better performance on wells with higher bottom hole pressure, located along the erosional axes formed during the deposition period, thus providing the zone with higher oil saturations. The wells with good responce also showed to have cleaner type of sand with less clay associated to it. This technology has improved the overall production of the reservoir and it has being proved to be cost effective. It is currently being used as a conventional treatment in the Tia Juana District of Western PDVSA.