Trisomie inheritance of the nucleolar chromosome (chromosome 6) in maize is the object of this investigation. The meiotic behaviour of the three chromosome 6 units was observed in microsporocytes, while the female progeny of trisomie individuals was examined in root tips. The outcome of this analysis suggests that meiotic losses of a supernumerary chromosome 6 do not explain, alone, the failure of trisomic types to reach the theoretical frequency of 50% of the progeny. Post-meiotic losses, at the expense ofn +1 spores and/or 2n+ 1 zygotes or embryos may therefore contribute to this failure. The same conclusion is reached by analysing the microspore quartets with regard to their nucleolar make-up. The frequency of a peculiar quartet showing two spores with two nucleoli in diagonal position suggests that the daughter units originating from pre-division of univalents have a tendency to be included in telophase nuclei of the second meiotic division, instead of being lost. A difference found regarding the size of nucleoli in spores with one or two nucleolar organizers suggests that these regions are not autonomous or independent of each other, but somewhat regulated by the cell's needs.