Phytotoxicity of oil shale semi-coke has been studied by germination bioassay. Timothy (Phleum pratense) seeds were germinated on solid semi-coke and in its water extracts. Fresh send-coke completely blocked seed germination but weathering significantly decreased its inhibition effect. The inhibitory effect of aqueous extracts was less than that of solid semi-coke, and it decreased as the liquid-to-solid ratio used for preparing the extracts increased. The inhibition of seed germination was mainly caused by high content of soluble salts in semi-coke. Good correlations were found between seed germination and electrical conductivity, and Ca(2+), content of water extracts.