Many poly(ferrocenyldimethylsilane) (PFS) block copolymers form fiber-like micelles with a semicrystalline core in selective solvents. Solvent effects on micelle formation are not well understood. This paper compares micelle formation for a sample of PFS50–PI1000 (the subscripts refer to the number-average degrees of polymerization) in decane with that in tert-butyl acetate (tBA), a more polar solvent. When micelle formation is seeded, by adding block copolymer as a concentrated solution in tetrahydrofuran to solutions of micelle fragments, micelle growth was similar in both solvents. Micelles with a narrow length distribution were formed and the length increased in proportion to the amount of polymer added. In contrast, when micelles were prepared by heating a sample of the block copolymer in decane or tBA to 90 °C and allowing the solution to cool, pronounced differences were observed. In decane, micelles with a uniform width (10 nm) and a length on the order of 5 μm formed after 1 h, and grew to about 10 μm after 5 days. In tBA, aliquots taken from solution 1 h after cooling appeared to undergo microphase separation only upon solvent evaporation. Ribbon-like structures were observed after 1 and 5 days aging, but these evolved into fiber-like structures with a uniform 10 nm width and lengths greater than 30 μm after 25 days. These differences observed in the rate of micelle formation likely reflect differences in the nucleation stage of micelle formation. tBA is a better solvent for the PFS block than decane. As a consequence, it appears to take much longer for semicrystalline micelle nuclei to form in tBA. The seeded growth experiments demonstrate that once seed micelles are present, growth occurs similarly in both solvents.