A flame retardant polyamide fiber is prepared from a composition containing specific amounts of a polyamide, a poly(phenylene ether)-polysiloxane block copolymer reaction product, a flame retardant that includes a metal dialkylphosphinate, and a compatibilizing agent. The composition used to form the fiber can be prepared in a process that includes melt blending a portion of the polyamide with all the other ingredients to form an intermediate composition, and then melt blending the intermediate composition with the remainder of the polyamide. The composition has small disperse phase particles that facilitate its use to melt spin fibers.