Technical Abstract: Several members of the crucifer family including white mustard, brown mustard, black mustard, leafy turnip, rapeseed and garden were examined for their potential as allelopathic green manure crops. Hemp sesbania germination and fresh weight was inhibited by chopped leaf tissues of all green manures tested, including when added to a sandy loam soil. Wheat seed germination was inhibited by white and black mustards and cress, although none of the treatments reduced fresh weight of surviving seedlings. The major volatiles released by chopped plants were determined by solid phase microextraction sampling and identified by gas chromatography-mass spectrometry. These included allyl isothiocyanate (allyl-ITC), 3-butenyl isothiocyanate, benzyl isothiocyanate (benzyl-ITC), cis-3 hexen-1-ol, and trans-2-hexenal. These compounds, together with methyl-ITC, beta-phenylethyl-ITC, benzaldehyde, beta-ocimene, and alpha farnesene were tested for inhibition of seed germination of several crop and weed species when applied as volatiles. Of these compounds, allyl and methyl isothiocyanates were the most inhibitory, completely inhibiting the germination of all species at a headspace gas concentration of 1 ppm in airtight glass containers. Selecting mustard green manures which release high levels of allyl-ITC would appear to be optimal for allelopathic activity, although plants which produce high levels of benzyl ITC also appear promising.