Developing embryos proved to be a suitable source of cells for advanced cytological investigations on Amphipods. Conventional karyotyping, Ag- and fluorochrome-staining, C-banding, endonuclease digestion, fluorescent in situ hybridization (FISH) and nuclear DNA flow cytometric assay were tested in the Ischyroceridae Jassa marmorata. The karyotype consists of 6 chromosome pairs of which 5 are metacentric and 1 subtelocentric. The rDNA/FISH revealed that major ribosomal cistrons are located on the telomeric regions in the short arm of pair 6. A marked size variation of hybridization signals was observed. Silver and fluorochrome staining enhanced no chromosome regions. Constitutive heterochromatin was scarce and limited to tiny, pericentromeric blocks as revealed by C-banding and three different restriction enzymes. The diploid genome size (1.90 pg) and the Adenine-Thymine base-pair nuclear DNA relative amount (28.85%) of J. marmorata are by far the lowest encountered in Amphipoda and Crustacea, respectively.