The optical density-temperature profile of double-stranded poly(A + U), triplestranded poly(A + 2U), and double-stranded RNA from rice dwarf virus in solutions with and without poly-clysine has been examined. When poly-clysine is added, more than one melting temperature T , is observed for poly(A + U) and poly(A + 2U). One of them is considered to correspond to the melting of the polynucleotide molecule free from poly-clysine, and another to the melting of a polynucleotide-poly-clysine complex. For rice dwarf virus RNA, the T , assignable to the complex is not found to be lower than 99ยฐC. I n every case, however, the hyperchromicity observed at the T , of the free polynucleotide molecule is lowered linearly as the amount of poly-clysine added to the solution increases. This fact is taken as indicating that there is a stoichiometric complex formed. The stoichiometric ratio lysine/nucleotide in each complex is determined by examining the relation between the amount of poly-clysine added to the solution and the percentage of hyperchromicity remaining at T , of the free polynucleotide molecule. The ratio is found to be 2/3 for all of the three complexes. A discussion is given on the molecular conformations of four types of polynucleotide-polylysine complex hitherto found: (A) double-stranded DNA plus poly-L-lysine in which the lysine/nucleotide ratio is 1, (B) three-stranded RNA (poly(A + 2U)] plus poly-clysine in which the ratio is 2 / 3 , (C) double-stranded RNA [poly(A + U) or rice dwarf virns RNA] plus poly-clysine in which the ratio is 2/3, and (D) double-stranded RNA [poly(I + C)] plus poly-tlysine in which the ratio is 1/2.