The paper studies toxicity of drilling and cementing fluids, used in offshore oil and gas operations, at concentrations of 10, 50, 100, 500 and 1000 mg/L for the mass species of benthic marine communities in the coastal part of the Black Sea: the eelgrass Nanozostera noltii and the amphipod Chaetogammarus olivii. The paper analyses effect of these toxic mixtures on the increase in biomass, leaves and roots of the eelgrass and on the survival of amphipods after 10, 20 and 30 days of exposure. Drilling fluid was shown to be more toxic than cementing fluid for the test organisms. Exposure to 10 mg/L of drilling fluid reduced the plant biomass growth by 49% after 10 days and by 62 and 78% after 20 and 30 days, respectively. With increase in the drilling fluid concentration to 50–100 mg/L, this indicator continued to decline rapidly to 60–80% relative to the control, and at a concentration of 500–1000 mg/L, the plants died. The roots of eelgrass were more sensitive to the toxicant than the leaves: the root growth showed a marked tendency to decrease by 48% relative to the control at toxicant concentrations of 50–100 mg/L after only 10 days. The harmful effect of the cementing fluid on the eelgrass was less pronounced than that of the drilling fluid. Exposed to the cementing fluid, the plants died at the highest concentration of the toxicant (1000 mg/L) after 30 days. No significant differences were found between the leaf growth in the test and control variants, but the root growth decreased significantly by 64 and 90% at 10 and 20 days at cementing fluid concentrations of 500 and 1000 mg/L, respectively. Throughout the experiment, the survival rate of the crustaceans exposed to over 10 mg/L drilling fluid was significantly lower than the control (30–85%). During exposure to the cementing fluid, however, significant differences were observed only at the highest concentration of 1000 mg/L. An ecotoxicological assessment of substances used in oil well drilling is necessary to determine their hazard when used in oil and gas production, as well as to select optimal components in their composition that contribute to reducing environmental damage to benthic marine communities.