A study was conducted to optimize the biodegradation in soil slurries of phenanthrene initially dissolved in nonaqueous-phase liquids (NAPLs). The slow rate of degradation of phenanthrene in dibutyl phthalate was increased by addition of phenanthrene-degrading microorganisms to soil slurries containing the NAPL. The rate was further increased and the acclimation phase was shortened if the inoculum was grown in a medium containing the hydrocarbon and the phthalate before addition to the slurries. Composition of the growth medium only shortened the acclimation but had no effect on the rate. Vigorous agitation increased the rate and extent of mineralization of phenanthrene in dibutyl phthalate. The effect of temperature was affected by the presence and identity of the inoculum. Rapid and extensive mineralization of phenanthrene initially present in hexadecane and diesel oil were attained by use of intense agitation of the NAPL/soil slurry and inoculation with microorganisms grown in the presence of the NAPLs, but the influence of these variables was less with other NAPLs. Vigorous agitation and addition of an inoculum 24 h after introduction of a nonionic surfactant enhanced biodegradation of phenanthrene initially in 150 Bright stock oil and dibutyl phthalate. The results suggest improved means for the bioremediation of sites contaminated with NAPLs.