Fiber-like micelles based on biodegradable and biocompatible polymers exhibit considerable promise for applications in nanomedicine, but until recently no convenient methods were available to prepare samples with uniform and controllable dimensions and spatial control of functionality. "Living" crystallization-driven self-assembly (CDSA) is a seeded growth method of growing importance for the preparation of uniform 1D and 2D core-shell nanoparticles from a range of crystallizable polymeric amphiphiles. However, in the case of poly(l-lactide) (PLLA), arguably the most widely utilized biodegradable polymer as the crystallizable core-forming block, the controlled formation of uniform fiber-like structures over a substantial range of lengths by "living" CDSA has been a major challenge. Herein, we demonstrate that via simple modulation of the solvent conditions via the addition of trifluoroethanol (TFE), DMSO, DMF and acetone, uniform fiber-like nanoparticles from PLLA diblock copolymers with controlled lengths up to 1 μm can be prepared. The probable mechanism involves improved unimer solvation by a reduction of hydrogen bonding interactions among PLLA chains. We provide evidence that this minimizes undesirable unimer aggregation which otherwise favors self-nucleation that competes with epitaxial crystallization from seed termini. This approach has also allowed the formation of well-defined segmented block comicelles with PLLA cores via the sequential seeded-growth of PLLA block copolymers with different corona-forming blocks.