We use Monte Carlo methods to investigate the asymptotic behaviour of the number and mean-square radius of gyration of polygons in the simple cubic lattice with fixed knot type. Let p n (τ ) be the number of n-edge polygons of a fixed knot type τ in the cubic lattice, and let R 2 n (τ ) be the mean square radius of gyration of all the polygons counted by p n (τ ). If we assume that p n (τ ) ∼ n α(τ )−3 µ(τ ) n , where µ(τ ) is the growth constant of polygons of knot type τ , and α(τ ) is the entropic exponent of polygons of knot type τ , then our numerical data are consistent with the relation α(τ ) = α(∅) + N f , where ∅ is the unknot and N f is the number of prime factors of the knot τ . If we assume that R 2 n (τ ) ∼ A ν (τ )n 2ν(τ ) , then our data are consistent with both A ν (τ ) and ν(τ ) being independent of τ . These results support the claims made