DSC and time-resolved WAXS and SAXS are used to study the structure development during isothermal crystallization of poly(glycolic acid) (PGA) in the temperature range 180-195 degrees C. It is shown that the crystallization rate increases with degree of supercooling in the temperature range of consideration. WAXS and DSC crystallinity measurements agree well and a final crystallinity of 50% is found independently of the crystallization temperature. In-situ SAXS measurements indicate that for PGA the final crystal thickness approaches a limiting value of 70 A independent of the crystallization temperature in the range 195-180 degrees C. The material develops a well-defined lamellar structure during crystallization at the highest crystallization temperature under study (195 degrees C). We show that by increasing the degree of supercooling it is possible to hinder the formation of the lamellar structure and crystals, resulting in a less ordered structure. We report that PGA fibers with elastic modulus in the range 20-25 GPa can be prepared by adequate control of the structure before solid-state plastic deformation. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1013-1018, 2009