A nucleolar substance adjacent to early prophasic sex bivalents is observed and studied by cytochemical means in certain broad-nosed male weevils. The substance soon disintegrates, possibly because of an active nucleolus organizer. By early diplotene, the sex bivalent assumes a parachute shape (Xyp). An argyrophilous, fibrous substance appears first at the inner borders of X and y, then invades the whole space between them. Most commonly used nucleolus and protein stains leave it unmarked. RNA and histones seem to be excluded. Only AgNOR staining marks the substance clearly, especially if a formalin-containing fixative has been used.