The DNA in sperm and certain viruses is condensed by arginine-rich proteins into toroidal subunits, a form of packaging that inactivates their entire genome. Individual DNA molecules were manipulated with an optical trap to examine the kinetics of torus formation induced by the binding of protamine and a subset of its DNA binding domain, Arg 6 . Condensation and decondensation experiments with λ-phage DNA show that toroid formation and stability are influenced by the number of arginine-rich anchoring domains in protamine. The results explain why protamines contain so much arginine and suggest that these proteins must be actively removed from sperm chromatin after fertilization.