Filament-like polymer crystal fibers are prepared from a solution of a crystallizable polymer, such as polyethylene or polypropylene, in a vessel containing a spinning rotor, preferably having a slightly roughened surface, according to the disclosed invention. The fiber thus formed is taken up and removed at a rate equal to the crystal growth rate. The longitudinal crystal growth rate is of sufficient speed for commercial application while at the same time yielding fibers of outstanding mechanical properties.