Acute toxicity of 12 coal derived liquids representing 3 different technologies and 4 different severities of hydrotreatment has been examined with theTetrahymenapopulation growth bioassay.Tetrahymenawere exposed to various concentrations of the organic materials and growth impairment was monitored. In addition, analyses of the major organic elements in the coal liquids are presented. Coal‐derived liquids have a greater heteroatom and aromatic content than do natural crude oils. Hydrotreatment, the catalytic addition of hydrogen, concomitantly reduces toxicity as well as heteroatom content and aromaticity. Regression analysis of log toxicityvs% weight of the major organic elements suggests hydrogen content may be a good indicator of relative toxicity of coal‐derived liquids.