The influence of Yarrowia lipolytica inoculum on biodegradation of hydrocarbons, and changes in microbiota composition in the soil contaminated with petroleum have been investigated. The material under study was contaminated clay soil, containing petroleum-derived substances at approximately 17 000 [mg/kg d. m.]. Microbiological analysis was carried out by the cultivation method and the content of individual hydrocarbons (n-aliphatic, BTEX and PAHs) was determined by the GC/MS method. The largest decrease of oil-derived substances, versus the control sample, was recorded at the beginning of the process. During the first 30 days, the yeast inoculation caused most effectively removal of n-aliphatic hydrocarbons and PAHs (approximately 80% reduction), however, the content of BTEX increased nearly three times. After 60 days of the process, PAHs concentration further decreased (by 40%), concentration of n-aliphatic hydrocarbons decreased a little, however the content of BTEX increased by 10%, compared to the initial concentration. Stimulating the biodegradation process with the yeast inoculum influenced the increase of the bacteria count, mainly Gram-positive, with simultaneous decrease of fungi number.