A new proteic killer gene system, hig, was identified on the plasmid Rts1. The hig locus consisting of a higA and higB is directly related to the temperature sensitive host cell growth conferred by Rts1. We proved that higB encoding presumably a 92-amino-acid polypeptide inhibited segregation of plasmid free cells, and higA encoding a 104-amino-acid polypeptide suppressed the higB function both in cis and in trans.