Aims: To isolate and characterize new marine bacteria capable of tolerating high concentrations of organic solvents, and to understand the toxic effects of these chemicals on marine bacteria. Methods and Results: Five marine bacteria able to tolerate 0.1% (v/v) toluene were isolated and characterized on the basis of their growth and survival rates in the presence of different organic solvents. The toluene-tolerant marine bacteria identified in this study could not grow in the presence of 0.1% (v/v) of several organic solvents with a log P-ow higher than that of the toluene (which in theory should be less toxic than toluene). The mechanisms underlying solvent tolerance were explored. Conclusions: Isolates of four different genera were identified as toluene-tolerant. Toxicity of a second phase of an organic solvent toward these isolates could not be predicted on the basis of the solvents' log P-ow. Significance and Impact of the Study: To improve the biodegradation rate of some water-insoluble compounds, double-phase bioreactors can be used. This type of bioreactor will require strains able to grow in a salt-containing environment and able to tolerate a second phase of an organic solvent.