Sinalbin was identified as a chemical component of insect anti-xenosis and antibiosis resistance mechanisms in seedlings ofSinapis alba by DEAE-Sephadex chromatography, HPLC, treatment with sulfatase and myrosinase, various feeding tests using artificial and natural substrates, and by measuring sinalbin concentrations in cotyledons and leaves during seedling development. The effects of sinaibin on feeding were dependent upon the insect species and upon the rapidly changing profile of sinaibin concentrations in the developing seedling. The high concentrations of sinalbin found in young cotyledons (up to 20 mM) and leaves (up to 10 mM) deterred the feeding of the flea beetle,Phyllotreta cruciferae Goeze and larvae of the bertha armyworm,Mamestra configurata Walker. The protection that sinalbin confers upon the vulnerable, newly emerged seedling (and upon tiny, young leaves) appears critical for the first few days of survival ofS. alba under feeding pressure from flea beetles in the field. The lower concentrations of sinaibin found in older cotyledons and leaves (2-3 mM) offer little or no protection againstP. cruciferae and may actually stimulate the feeding of this crucifer specialist. These concentrations of sinaibin, however, are still effective in reducing the level of feeding by larvae of the more generalist feederM. configurata.