In the present investigation chromosomal preparations of Asellus aquat
icus were sequentially stained with chromomycin A3 to reveal the heter
ochromatic areas, hybridized in situ with rDNA probes in order to map
the ribosomal genes and finally silver stained to check the transcript
ional activity of these genes. The results indicate the existence of a
substantial correspondence of location and size among the heterochrom
atic regions and the regions over which the in situ hybridization sign
als spread. The ribosomal genes, quite independently of their location
in the secondary constriction, can be silver stained and thus appear
to be transcriptionally active. The ribosomal sequences also hybridize
to the entire heterochromatic areas observed on the probable Y chromo
some identified in some males of a natural population. These rRNA gene
s are only rarely transcriptionally active.