The mechanical behavior of semicrystalline Nylon 11 was studied at strain rates between 10(-3) and 8800 s(-1). X-ray diffraction and DSC were employed to examine the crystal structure and the crystallinity content. The as-received material comprised a mixed structure of a predominately triclinic (a) form. DSC revealed that the material gave rise to two melting peaks. The compressive flow stress of Nylon 11 experienced a large increase at 1200 s(-1) and decreased at higher strain rates. The maximum level of the flow stress corresponded with a higher level of crystallinity and a structure mainly of a pseudohexagonal form. The subsequent drop in stress at higher rates was associated with a decrease in the crystallinity content and a mixed crystal structure, different from that observed in the as-received material. After compression, the low melting peak disappeared and the material melted over an increased temperature range.