Abstract. -We show that Kramers rate theory gives a straightforward, accurate estimate of the closing time τc of a semiflexible polymer that is valid in cases of physical interest. The calculation also reveals how the time scales of chain relaxation and closing are intertwined, illuminating an apparent conflict between two ways of calculating τc in the flexible limit.The looping of polymers is a physical process that allows contact and chemical reaction between chain segments that would otherwise be too distant to interact. Polymer loops are particularly important in biology: In gene regulation, looping allows a DNA-bound protein to interact with a distant target site on the DNA, greatly multiplying enzyme reaction rates [1,2]. Similarly, DNA looping in the 30-nm chromatin fiber may trigger the initiation of DNA replication at different sites along the DNA by enabling long-distance interactions [3]. In protein folding, two distant residues start to come into contact via looping [4,5]. Measurements of loop formation in single-stranded DNA segments with complementary ends have also been used to extract elasticity information (e.g., the sequence-dependent stiffness of single-stranded DNA [6]).Despite its importance and despite considerable theoretical effort, there are relatively few analytical results concerning the dynamics of loop formation. Even for the simplest case of an ideally flexible polymer with no hydrodynamic effects (or simply a Rouse chain), there are two rival theoretical approaches that lead to contradictory results: Szabo, Schulten, and Schulten (SSS) conclude that the time for a loop to form ("closing time" τ c ) should scale for moderately large polymer lengths L as. The discrepancy between the two continues to spur debate [10,11]. For the important case of stiff chains [12,13], where the polymer length L is not too much longer than the persistence length ℓ p , only limited numerical results are known [see, for example, [14,15] and references therein]. The main difficulty arises from the interplay