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Moav, Rom(Hebrew U., Jerusalem), and D. R. Cameron. Genetic instability in Nicotiana hybrids. I. The expression of instability in N. tabacum × N. plumbaginifolia.Amer. Jour. Bot. 47(2): 87—93. 1960.—N. tabacum(n = 24) and N. plumbaginifolia(n=10) are distantly related species both from morphological and cytological points of view. Hybrids of these species with various genome dosages have exhibited somatic variegation when plumbaginifoliadominant characters were superimposed on an appropriate tabacumgenetic background. Five loci were studied in this respect: Whand Tg—for flower coloration; Ws—for chlorophyll production; Kl—for pollen abortion and Bs—for black shank resistance. All 5 were found to be unstable. Backcross progenies of the sesquidiploid hybrid (tbc‐tbc‐pbg)to tabacumshowed a marked increase in intensity of variegation. This has been attributed to the breaking up of the plumbaginifoliagenome into individual chromosomes. The evidence indicates that variegation was due to somatic chromosomal aberrations which probably characterized all the plumbaginifoliachromosomes. An hypothesis regarding the heterogeneity of F1hybrids of distantly related homozygous species is outlined and the occurrence of instability due to hybridization in other Nicotianahybrids is discussed.