The crystallization behavior of a series of ethylene-b-(3-methyl-1-butene) diblocks (E/MB) is investigated via simultaneous, time-resolved small-angle and nide-angle X-ray scattering. All the diblocks contain 26-27 wt % E, but possess varying molecular weights and hence varying segregation strengths in the melt, ranging from homogeneous to strongly segregated. Crystallization from homogeneous or weakly segregated melts at typical rates yields a semicrystalline solid consisting of alternating E and MB lamellae. By contrast, a strongly segregated diblock shows crystallization within the E cylinders that are present in the melt, even for slow crystallization rates and at temperatures where both blocks are well above their glass transition temperatures. Modest differences in intercylinder spacing between the melt at high temperatures and solid at low temperatures are shown to arise from the temperature dependence of the intercylinder spacing in the melt. Because chain diffusion in these block copolymer melts is not always fast on the crystallization time scale, the value of the melt spacing that is "frozen in" upon crystallization depends on the specimen's thermal history. Despite the marked differences in the final structure between polymers where crystallization is constrained to cylinders and those where it "breaks out" to form lamellar microdomains, the kinetics are not remarkably different, at least in these polyethylene-based systems.