The objective of this research was to identify the major sources of methanogenic toxicity in hemp black liquors. Hemp extractives were shown to be highly toxic. The stem wood extractives, which had a 50% inhibitory concentration (IC) corresponding to 0.25 g COD/l, were somewhat more toxic than the bark extractives with a 50% IC corresponding to 0.65 g COD/l, The methanogenic toxicity of black liquors from hemp was comparable with the toxicity of black liquors from woody feedstocks and ranged between 2.8 and 4.8 g COD/l. Removal of extractives from the hemp fiber prior to pulping only partly reduced methanogenic toxicity, in spite of high extractive removal efficiencies. The residual extractives remaining in the feedstocks after fiber extraction (10 to 30% of the extractives according Soxhlet determination) were apparently released into the black liquor during pulping. To improve removal of the extractives, hemp black liquor was subjected to several detoxification treatments. Ethyl ether extraction and Amberlite XAD-2 treatment achieved complete wastewater detoxification. These results indicate that the apolar extractive fraction is the major source of methanogenic toxicity in hemp black liquors.