The effects of fresh-prepared, 4-day-aged and 8-dayaged Ho. 2 fuel oil water soluble fractions on the growth of Selenastrum capricornutum were investigated. The freshprepared water soluble fraction inhibits both the growth rate and cell yield of S. capricornutum. Its toxicity decreases after four days but there is still residual toxicity. This residual toxicity decreases very slowly; the effects of the 8day-aged water soluble fraction on S. capricornutum are comparable with those of the 4-day-aged one. The components of the water soluble fractions were identified by a mass spectrometer interfaced to a gas chromatograph. The major components are aromatic compounds, of which, the benzene and indene derivatives volatilize '.very quickly. Naphthalene and methyl naphthalenes volatilize in four days. However, the di-aromatic and tri-aromatic compounds, having boiling points higher than naphthalene,are still in the medium after 8 days. Sixteen component compounds were chosen for chemical analysis and toxicity testing. The concentrations of these compounds in the fresh-prepared , 4-day-aged and 8-day-aged water soluble fractions were determined, and their toxicities were investigated by "algal lavra" technique. The concentration distribution test on the agarized medium shows that the concentrations of naphthalene and phenanthrene are comparable at a selected distance after diffusing in the medium for four days. The results indicate that methyl, dimethyl and trimethyl naphthalenes are more toxic than naphthalene. The most toxic compounds are dibenzofuran, fluorene, phenanthrene and dibenzothiophene, v/hich have high boiling points.