Abstract: Low attenuation, all-plastic optical fiber is made by using starting materials which are rigorously purified to remove light-absorbing impurities and solid particles of dirt, polymerizing the core monomer under pressure with a heating schedule which leads to at least 98% conversion to polymer as a cylindrical preform, and ram extruding the core of the optical fiber while melting only the forward end of the preform of core polymer so as to preclude contamination of the core by dirt or formation of bubbles by polymer degradation. Optical fibers so made exhibit light attenuations as low as 300 dB/km or lower.