We report the formation of near uniform block copolymer fiberlike micelles of controlled length with a crystalline polyselenophene core via the seeded growth process termed living crystallization-driven self-assembly (CDSA). Poly(3-decylselenophene)-block-poly(dimethylsiloxane) (P3DSe-b-PDMS) forms long, fiberlike micelles in selective solvents. Attempts to control the length of these fibers by seeded growth following sonication to form the seeds afforded samples with lengths limited to ca. 300 nm. However, we found that direct dispersion of P3DSe-b-PDMS in diethyl ether yields short and relatively low-dispersity nanofibers (Ln = 41 nm; Lw/Ln = 1.16) which can be used as seeds. In this case, seeded growth led to low-dispersity fibers with length control up to ca. 900 nm. Moreover, we demonstrate that block co-micelles with spatially distinct PDMS and polystyrene (PS) coronal segments can be accessed from the sequential addition of dissolved P3DSe-b-PS to preformed P3DSe-b-PDMS micelles.