Abstract: The biotoxicity of five polycyclic aromatic hydrocarbons(PAHs)was assayed by Microtox test．It has been found that some kinds of PAk such as naphthalene．phenanthrene and fluoranthene have acute toxicity to the test microbes in the water solution produced using DMSO solvent and the toxicity is strengthened with the increasing concentration．The toxicity of phenanthrene is stronger than that of naphthalene at the same concentration．The inhibitory effect of luminosity in the solution of naphthalene is up to 100％ under its solubility and the EC₅₀ value of naphthalene is 4．32mg／L．The inhibitory effect of luminosity in the solution of phenanthrene and fluoranthene is only up to 50％ and 15％ at the concentration near its solubility，respectively．However，anthracene and pyrene have no effect on the microbes even at their highest concentration．It appears that the sensitivity of the detection of the toxicity of PAi ls by Microtox test is restricted to their low solubility，and the toxicity observed in the leachates of PAHS contamination is linked to the low molecular PAHs such as naphthalene and phenanthrene．