The second virial coefficient A(2) was determined for atactic oligo- and polystyrenes in methyl acetate below, at, and above Theta (41.5 degrees C) in the range of weight-average molecular weight M-w from 4.74 x 10(2) (tetramer) to 7.72 x 10(6). The gyration-radius expansion factor alpha(s) was also determined for the sample with M-w = 7.72 x 10(6). It is found that A(2) depends on M-w appreciably in the range of small M-w. Although the dependence of A(2) On M-w in methyl acetate is different from that previously found in cyclohexane, the former may also be explained quantitatively by the Yamakawa theory that takes account of the effect of chain ends, indicating that the difference in the M-w, dependence between A(2) in the two Theta solvents arises from that between the effects of chain ends. An analysis gives values of the effective excess binary-cluster integrals beta(1) and beta(2) associated with the chain end beads and also the binary-cluster integral beta between intermediate identical beads as functions of temperature T, all of them except for beta above Theta being quadratic in tau = 1 - Theta/T. With these values of beta, the conventional and scaled excluded-volume parameters z and (z) over tilde below Theta are calculated. The results for the part A(2)((HW)) of A(2) without the effect of chain ends along with those previously determined in cyclohexane give a single-composite curve below Theta when A(2)((HW)) M-w(1/2) is plotted against z, being consistent with the two-parameter theory prediction irrespective of the difference in solvent condition. This is in contrast to the behavior of A(2)((HW)) above Theta. It is also found that if alpha(S) is plotted against (z) over tilde (or z for large M-w), the present data points along with those in cyclohexane form a single-composite curve, indicating that the quasi-two-parameter theory is valid for as below Theta as well as above Theta irrespective of the difference in solvent condition.