Materials used in this study consisted of small and large seeded flax varieties Redwing and Beta 210, and of their reciprocal crosses and backcrosses. The seed weight means (mg's/seed) of reciprocal crosses were the same in F1 but significantly different in F2 and F3 generations indicating thus their nonequivalence with respect to this character. This nonequivalence was detectable also in the backcross reciprocals. On the assumption that Redwing and Beta 210. have different plasmatypes and the hybrid has the same plasmatype as its female parent, the available 24 families, including the parents as selfs, were grouped into two genomically the same but plasmatically supposedly different sets. A detailed analysis of the family means in these two sets led to the conclusion that the inheritance of the character considered was rather complicated. In F1 generation the Beta 210 set of genes was partially dominant over its allelic Redwing set ([h]<[d]). In the subsequent generations the Beta 210 set of genes has sustained a certain degree of loss of expressivity. This loss was 73% in the plasmatypically Redwing set of families and 20% in the plasmatypically Beta 210 set of families. This difference in the loss of expressivity and the gene-dosis effects, detected mainly in the plasmatypically Beta 210 set of families, indicated that the nature of the reciprocal cross nonequivalence observed in this study was both cytoplasmic and nuclear.