The article presents the results of experiments on the ability of the green alga Ulva lactuca to absorb and transform diesel fuel from marine water for 5 and 10 days. The original marine water contained 0.62 mg/L of petroleum hydrocarbons, which is about 12 maximum permissible concentrations (MPC). During the experiment with the addition of 20 mg/L of diesel fuel (400 MPC) to the water, the absorption of the introduced hydrocarbons was observed in the experimental tanks without algae. Apparently, they were absorbed by water microorganisms. On the 5th day of the experiment, the petroleum hydrocarbons concentration in the water decreased by 40% and amounted to 12 mg/L (240 MPC). When ulva thalli were added to the water, the total content of petroleum hydrocarbons in the water on the 5th day decreased by 86% (to 2.8 mg/L), and on the 10th day, it increased (to 4.2 mg/L). A slight increase in the concentration of diesel fuel hydrocarbons in water indicates a reverse process of releasing hydrocarbons absorbed by ulva into water. In the experiment with the addition of diesel fuel to the water at a concentration of 10 mg/L, the content of hydrocarbons in algae tissues on the 5th and 10th days was recorded at the level of 0.6 mg/g. The marker ratio of Σ n-alkanes / Σ petroleum products in ulva during the experiment was 0.2. A decrease in this indicator to 0.18 on the 10th day of the experiment indicates the beginning of the transformation of the hydrocarbons chemical structure. When 20 mg/L of diesel fuel (400 MPC) were added to the water, this indicator on the 5th and 10th days was 0.25 and 0.28, respectively, indicating an active process of hydrocarbon absorption by the algae surface, which was not yet complete by the 10th day. The experiment results allow us to conclude that U. lactuca is able to absorb and transform petroleum hydrocarbons and participates in the bioremediation of coastal waters.