Studies of the thermal properties and lamellar morphology of a highly structurally regular fraction of a Ziegler-Natta type isotactic poly(propylene) have been carried out. This fraction has an isotacticity content of mmmm = 0.995 and a molar fraction of defects of 0.001. It is thus, among the most structurally regular isotactic poly(propylene) samples whose properties are reported. Differential scanning calorimetry as well as electron and optical microscopy were used to characterize the specimens. The fraction was crystallized from the melt over a very wide range of crystallization temperatures (135 degrees C less than or equal to T-c less than or equal to 167 degrees C). Monoclinic, a type crystals were formed over the whole crystallization range. The formation of cross-hatching, or lamellae branching, was also observed over the complete interval of crystallization temperatures. The formation of the tangential 'daughter' lamellae at temperatures greater than 160 degrees C can be attributed to the high structural regularity of the fraction studied. Relatively low crystallization temperatures (130 degrees C to 150 degrees C) show extended regions of woven lamellae having similar thicknesses with occasional groups of parallel long radiating lamellae. A morphology of rather thick long radiating lamellae and thin, transverse lamellae is formed at temperatures greater than or equal to 160 degrees C, The angle between the daughter and mother lamellae of approximately 100 degrees is in agreement with crystallographic predictions. The two endotherms that are observed by differential scanning calorimetry can be identified with the melting of the two distinct lamellae populations. It is consistent with the optical microscopy observations where a change in the sign of the birefringence is observed on the melting of the daughter lamellae, When formed at relatively high temperatures (T-c > 160 degrees C) the mother lamellae subsequently melt at temperatures > 180 degrees C.