Forty-six species of poplar were introduced from home and abroad.Based on our observation of their three year's growth and our investigation on their resistance, several species were chosen which possessed resistivity of rust, and whose growth-speed was similar to that of Populus canadensis.Among the chosen species were Populus sercerouge and P.275 which were possessed of resistance, and P.gelrica, P.canadensis, P.robusta, P.15A which were subjected to slight infection. Out of fifteen species of poplar growing in the northeast area, we composed fiftyeight hybridized combinations, among four combinations of which we picked out such hybrid plants as capable of resisting Melampsora larici-populina Kleb.and M.Magnusiana Wagner.Through our test, the results are shown below: 1.Whether the species of Populus Sect.Aigeiros Duby.and Populus Sect.Tacamahaca Spach were crossed with each other or respectively between themselves, among their hybrids, there were no plants which could resist rust. 2.But when P.davidiana Dode was crossed with the species of Populus Sect.Aigeiros Duby.or with the species of Populus Sect.Tacamahaca Spach, among their hybrids appeared such plants in each type that possessed resistivity of rust. For instance, P.davidiana Dode×P.berolinensis; Populus davidiana Dode×P.manshurica Nakai; P.simonii Carr.×P.davidiana Dode; and P.berolinensis×P.davidiana Dode.and it was found that when P.davidiana Dode was taken as maternal plant, the proportion of hybrid plants with resistivity of rust was great in number, while in the case of other way round, the proportion small. We have only made a preliminary study of the relations between rust infection and leaf form and between rust infection and biochemistry.On account of one year's test only, no conclusion has been arrived at yet.