A consistent diploid number of 2n = 36 was determined for the sea urchin Paracentrotus lividus from the Gulf of Palermo by analysis of mitotic chromosomes of both early developing embryos and male gonads. The haploid number n = 18 was determined by counts of spermatocyte bivalents at diakinesis. A heteromorphic chromosome sex mechanism of the XY type is likely present in this species. This is indicated by the occurrence of a chromosomal pair, pair No. 2, which is heteromorphic in both morphology and size in about 50% of the mitotic figures (metaphases and anaphases) of embryos. In addition, heteromorphism of the same pair of chromosomes occurred during spermatogonial metaphases in the five male specimens investigated here. The detection of a low chromosome number (2n--36) compared to other echinoids (2n = 42 to 44), a heteromorphic chromosome sex system and the involvement of three chromosome pairs in nucleolar organization (NORs) provide evidence of the specialization of the P. lividus karyotype.