A comparative study of fungitoxicity and phytotoxicity was made with an homologous series of N-n-alkylethylenethioureas ranging from ethyl to dodecyl. Water solubility decreased and the oil/water partition coefficient increased with increasing length of alkyl side chain. Fungitoxicity increased with increasing chain length up to and including the octyl homologue followed by a sharp decline in the higher members of the series. The more fungitoxic homologues had the same mode of action and the cutoff in toxicity was due to limiting water solubility.The peak of maximum phytotoxicity depended on the availability of the compounds to the plant cells. As the series was ascended the mobility of the homologues in the plant was found to decrease. When the compounds were readily available to plant cells phytotoxicity increased as the series was ascended up to at least the octyl homologue, but when barriers were present, a peak for phytotoxicity was exhibited near the amyl homologue.Studies on the uptake of the compounds from aqueous solutions by potato tissue andMonilinia fructicolaspores showed that a distribution equilibrium existed between the tissue or spores and the external solution. With increasing length of side chain the affinity for the biophase increased and hence the differential toxicity could be attributed to the partitioning of the chemical between the two phases.