Differential thermal analyses of many 66 nylon samples have shown two endotherms in the melting range. This effect occurs with drawn nylon yarn, as previously observed by White, and also with annealed and precipitated nylon samples. Methods for the preparation of these samples are reported. At 242°C. the development from the melt of crystalline material which gives two melting peaks is associated with annealing rather than with the primary crystallization process. Previous observations of multiple melting peaks with other polymers are discussed.