The plant parasitic nematode, Tylenchus dipsaci Kühn, commonly known as the stem eelworm, or alternatively when occurring in narcissus, the bulb eelworm, is a major pest of a wide range of cultivated plants. Accurate knowledge concerning the detailed life-history of the nematode is still of limited extent, despite the large numbers of workers who, throughout Europe and more recently North America, have devoted much time to a study of the subject. In particular it is clear that much remains to be learned concerning the “biologic strain” theory. Investigators, probably without exception, agree that a large number of, so-called, biologic strains of the worm occur. Each of these strains, while morphologically identical with the others, appears to be restricted more or less rigidly to a particular species of host plant.