Abstract: Silicon nitride fibers having a carbon content of 5 weight percent or less have high heat insulating properties and favorable mechanical properties and are useful as heat resistant, highly insulating materials or reinforcing agents for composite materials, particularly in the aerospace industry. Such high purity silicon nitride fibers are produced by spinning a solution of perhydropolysilazanes, an inert solvent and a spinning agent into fibers, preferably continuous fibers, and firing the spun fibers at 800.degree. to 1300.degree. C. in an inert atmosphere to form silicon nitride fibers of high purity.