The morphological changes during the quasi-isothermal melt crystallization of a linear polyethylene sample at 126 degreesC were monitored by synchrotron WAXD and SAXS and compared to TMDSC data. A blocklike temperature-profile with an amplitude of 1 degreesC and a period of 2 min was applied. AFM at room temperature reveals lamellar crystals with lateral dimensions of several micrometers that are composed of grains with dimensions comparable to those based on WAXD peak width analysis. According to the X-ray data all reversible melting. and crystallization phenomena, probed by TMDSC, take place at the fold surface of the lamellar crystallites. There are no reversible changes at the crystal grain boundaries nor at the lateral surfaces of the lamellae, i.e., the crystal growth faces. The crystal growth rate, however, is affected by the temperature modulation.