The karyotype ofRoncus gestroiandRoncus belluatiiis reported.Roncus gestroihas a complement 2n= 43–44, having a chromosome system of sex determination of X0 type, andRoncus belluatiihas a complement 2n= 22. The possible relationships with other species of the same genus are analyzed and discussed together with some hypothesis on the general mechanism involved in the karyotypic evolution of the whole group.