Two new chromosomal races (V and VI) of the Steropleurus martorelli complex, as well as the corresponding natural interracial hybrids, have been found in a region in southern Spain. Hybrids can be easily distinguished because they are heterozygous for a large L2 pair (submetacentric–telocentric), which in addition carries the major nucleolar organizer region (NOR). Silver staining reveals that the chromosomal rearrangement involved in the origin of these two chromosomal races does not affect the expression of the major active NOR in the parental homozygous races, but a nucleolar dominance phenomenon can be detected in the heterozygous hybrids, which tends to suppress NOR activity in the telocentric L2 chromosome from race VI.Key words: Steropleurus martorelli complex, nucleolar organizer region activity, nucleolar dominance, interracial hybrid.