The mean-square radius of gyration (S-2) was determined from small-angle X-ray and light scattering measurements for 20 samples of atactic oligo- and poly(alpha-methylstyrene)s (a-P alpha MS), each with the fraction of racemic diads f(r) = 0.72, in the range of weight-average molecular weight from 6.48 x 10(2) (pentamer) to 3.22 x 10(6) in cyclohexane at 30.5 degrees C (Theta). The ratio (S-2)/x(w) as a function of the weight-average degree of polymerization x, first increases steeply for x, s 20, then passes through a flat hump at x(w) similar or equal to 40, and finally approaches its asymptotic value for x(w) greater than or similar to 1 x 10(3). On the basis of the helical wormlike (HW) chain model the HW model parameters are lambda(K0)(-1k) = 3.0, lambda(tau 0)(-1) = 0.9, lambda(-1) = 46.8 Angstrom, and M-L = 39.8 A-l, where Kg and to are the differential-geometrical curvature and tortion, respectively, of the characteristic helix of the HW chain taken at the minimum zero of its elastic energy,;l-l is the stiffness parameter, and ML is the shift factor defined as the molecular weight per unit contour length of the chain. The local chain conformations of a-PaMS are discussed in detail on the basis of the above values of the model parameters and compared with those of atactic polystyrene (f(r) = 0.59), atactic poly(methyl methacrylate) (a-PMMA) (f(r) = 0.79), and isotactic PMMA (f(r) approximate to 0.01) previously studied. It is then concluded that the a-PaMS chain tends to retain large and clearly distinguishable helical portions in dilute solution like the a-PMMA chain, as was expected from their chemical structures, although the helical nature of the former is somewhat weaker. To illustrate the situation, pictures are given of representative instantaneous contours of HW Monte Carlo chains for the above four polymers.