A mammoth mutant which produced about 4 more leaves than normal plants was obtained from a fluecured tobacco tariety, Hicks Broadleaf. It was found that selfed progeny of this mutant segregated into two types, normal plants with the same leaf number as Hicks Broadleaf, and mutant plants with the same leaf number as the parent. In spite of selection during four generations, these mutant plants could not be fixed. The genetic studies with this mutant are summarized as follows, 1) Selfed mutant plants segregated 2 : 1 for normal and mutant plants, while normal segregates producing only normal progenies. 2) F1 families of crosses between three normal varieties and mutant plants as well as crosses between normal and mutant individuals segregated 1: 1 for normal and mutant plants. 3) Photoperiodic response which greatly influen ced determinations of leaf number was different in normal and mutant plants ; normal plants were dominant homozygotes and the mutant plants were heterozygotes for a single factor coutrolling this characteristfc. 4) It was observed that approximately 25% of the seeds produced in the capsulcs of mutant plants were sterile. These capsules of mutant plants were sterile. These sterile seeds might be considered recessive homozygotes. 5) It was suggested that this zygotic lethal action might be due to a lethal gene almost completely linked with a gene controlling photoperiodic response.