Naphthalene is one of the polycyclic aromatic hydrocarbons (PAHs) that compose fossil fuels. The degradation of this compound in nature mostly occurs through the activity of soil microorganisms. These microorganisms are prevalent in locations contaminated with oil. The volcanic mud in Renokenongo Village, Sidoarjo contains PAHs caused by drilling activity. The aim of this research was to determine the biodegradation ability of naphthalene by Pseudomonas aeruginosa DRK 9.1 which was isolated from volcanic mud in Renokenongo Village. P. aeruginosa DRK 9.1 was grown in Bushnell- Haas medium with the addition of 0.02 % (w/v) naphthalene as a sole carbon source. The number of Colony Forming Units (CFUs) was determined using Total Plate Count (TPC) method and naphthalene degradation was evaluated using the High- Performance Liquid Chromatography (HPLC). The results showed that after 96 h of incubation, the total cell number increased from 3.96 x 109 CFU/mL to 3.08 x 1010 CFU/mL and naphthalene concentration decreased by 70.87 %.