This study describes the karyology of an isopod, Saduria entomon, and provides for the first time evidence of a XY/XX heterogamety for this species. The presented karyotype [56 (m sm ) 2 st XX/XY] is characterised by the dominance of metacentric and submetacentric chromosomes and is considered as apomorphic. This is the first time a valviferan is described with an XX/XY system. Mitotic and meiotic heterogamety observed in S. entomon suggests more advanced evolution of sex determination within this species in comparison to other Isopoda. Based on Ag-staining the presence of secondary constrictions and satellites on the 22nd pair of homologous chromosomes was identified. Inter- and intra-individual variability in chromosome number (2n  =  58, 2n  =  59 or 2n  =  61) was also observed. The between-species variation in heterogamety in isopods and the within-species variation in chromosomes numbers are discussed.