The dimensional features of an isotactic poly(lactic acid), poly(D-lactide) (PDLA), chain with a weight-averaged molar mass (M-w) ranging from 9.84 X 10(2) to 4.44 X 10(5) g mol(-1) are studied in tetrahydrofuran (THF) at 25 degrees C using static light and small-angle X-ray scattering and the intrinsic viscosity ([eta]). Sixteen PDLA samples with polydispersity indices (M-w/M-n) ranging from 1.04 to 1.15 are prepared by ring-opening polymerization of D-lactide, followed by fractionation using recycling preparative size-exclusion chromatography. The M-w dependence of the z-averaged root-mean-square radius of gyration (< S-2 >)(z)(1/2)) and [eta] is tabulated and compared with that of atactic poly(lactic acid) (PDL50, a random copolymer with a 1:1 molar ratio of D-lactide to L-lactide). The My, dependence of < S-2 >(1/2)(z) and [eta] and the scattering form factor [P(q)] for the PDLA chain are consistently and quantitatively described by a cylindrical wormlike model with a stiffness parameter (lambda(-1) = 6.5 nm), molar mass per unit contour length (M-L = 350 g mol(-1) nm(-1)), protruding effects at both ends (delta = 0.4 nm), bead diameter (d(B) = 1.06 nm), and excluded volume strength (B = 0.50 nm). The results demonstrate that the isotactic PDLA chain in THF at 25 degrees C possesses a considerably different conformation from that reported to date, that is, 10(3) or 3(1) helices in the crystalline state of poly(L-lactide) and the conformation of the atactic PDL50 chain (lambda(-1) = 2.9 nm, M-L = 270 g mol(-1) nm(-1), and d(B) = 0.62 nm) in THF at 25 degrees C. It is concluded that the stereoregularity has a significant effect on the conformation, and the PDLA chain has, in solution, a helical-like conformation which is stiffer and thicker with a larger radius of curvature and lower pitch than the PDL50 chain.