Direct nonradiative energy transfer (DET) experiments were carried out on five polyisoprene-poly(methyl methacrylate) diblock copolymers with polyisoprene volume fractions ranging from 0.07 to 0.5 (polyisoprene microdomains dispersed in a poly(methyl methaerylate) matrix). These experiments yield the ratio R/delta of the microdomain size R to the thickness of the domain-boundary interface delta as a function of the block copolymer overall chain length N. A plot of ln(R/delta) vs ln N indicates that the microdomain size varies as N-0.65, close to the N-2/3 predicted by theory, while the interfacial thickness delta (26 Angstrom) is independent of the degree of polymerization covered in this work. In addition, the Flory-Huggins interaction parameter chi for PI-PMMA was determined from the DET data (0.077). We conclude from these experiments that PI-PMMA belongs to the class of strongly segregated systems.