When eggs of the trichostrongylid nematode Haemonchus contortus were exposed to thiabendazole, the concentration required to prevent hatching in 90% of the eggs (MIC₉₀) was found to be 0.1 μg/ml (using 1% dimethylsulfoxide [DMSO] as solvent). In contrast, eggs of the free-living rhabditid nematode Caenorhabditis elegans hatched at normal rates at a concentration 200 times higher, i.e., 20 μg/ml, and showed only a partial inhibitory effect at a concentration 1,200 times higher, i.e., 120 μg/ml (in 3% DMSO). Because solubility limitations precluded the testing of higher concentrations of thiabendazole, a more soluble derivative, 5-([1-methylethoxy]carbonylamino)-2-(4-thiazloyl)-1H-benzimidazolyliminoacetic acid N,N-diethylethanamine salt, was tested against C. elegans eggs. The MIC₉₀ was found to be 400 μg/ml, and although the derivative was not tested against H. contortus eggs, this finding further suggests that C. elegans eggs have an exceptionally low degree of benzimidazole sensitivity.