The rod orientation and crystalline packing of a model semiconducting rodlike polymer, poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (DEH−PPV), is shown to affect the self-assembly of weakly segregated rod−coil block copolymers. The in-plane packing of DEH−PPV rods in lamellar poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene-b-isoprene) (DEH−PPV-b-PI) diblock copolymers is nearly identical to that observed in DEH−PPV homopolymers for compositions ranging from 0.42 to 0.82 vol % coil block. The crystal structure of DEH−PPV, characterized by grazing incidence X-ray diffraction and electron diffraction, consists of a tetragonal unit cell having c = 0.665 nm with a = b = 1.348 nm. The polymer chain axis is aligned along the [001] direction, and the nearest neighbor rod−rod spacing along 〈110〉 is 1.0 nm. As-cast thin films of DEH−PPV homopolymer demonstrate chain alignment primarily perpendicular to the substrate in 5100 g/mol homopolymer, while for 3500 g/mol homopolymer the chains align both perpendicular and parallel to the substrate. For the DEH−PPV-b-PI block copolymers, a sharper 001 reflection is observed due to the effect of microphase separation, improving alignment and stacking of the rods. The lamellar phases have a smectic A-like packing structure with the rods oriented parallel to the lamellar normal regardless of coil fraction; however, at coil fractions above about 0.8 the crystalline lattice of the rods becomes rapidly disordered as evidenced by loss of all but the two strongest Bragg reflections. This suggests that the constraints of packing into the unit cell outweigh the chain stretching and segment−segment repulsion energies that are predicted to lead to a transition from normal (smectic A) to tilted (smectic C) rod orientation within the lamellae at high coil fraction; increasing coil fraction breaks apart the crystalline lattice rather than distorting it into a tilted polymorph.