Exact two-spinon dynamic structure factor of the one-dimensional s=1/2 HeisenbergIsing antiferromagnet. Physical Review B, 57(1998) The exact two-spinon part of the dynamic spin structure factor S xx (Q, ) for the one-dimensional sϭ1/2, XXZ model at Tϭ0 in the antiferromagnetically ordered phase is calculated using recent advances in the algebraic analysis based on ͑infinite-dimensional͒ quantum group symmetries of this model and the related vertex models. The two-spinon excitations form a two-parameter continuum consisting of two partly overlapping sheets in (Q, ) space. The spectral threshold has a smooth maximum at the Brillouin zone boundary (Qϭ /2) and a smooth minimum with a gap at the zone center (Qϭ0). The two-spinon density of states has square-root divergences at the lower and upper continuum boundaries. For the two-spinon transition rates, the two regimes 0рQϽQ ͑near the zone center͒ and Q ϽQр /2 ͑near the zone boundary͒ must be distinguished, where Q →0 in the Heisenberg limit and Q → /2 in the Ising limit. In the regime Q ϽQр /2, the two-spinon transition rates relevant for S xx (Q, ) are finite at the lower boundary of each sheet, decrease monotonically with increasing , and approach zero linearly at the upper boundary. The resulting two-spinon part of S xx (Q, ) is then square-root divergent at the spectral threshold and vanishes in a square-root cusp at the upper boundary. In the regime 0ϽQ р /2, in contrast, the two-spinon transition rates have a smooth maximum inside the continuum and vanish linearly at either boundary. In the associated two-spinon line shapes of S xx (Q, ), the linear cusps at the continuum boundaries are replaced by square-root cusps. Existing perturbation studies have been unable to capture the physics of the regime Q ϽQр /2. However, their line-shape predictions for the regime 0рQϽQ are in good agreement with the exact results if the anisotropy is very strong. For weak anisotropies, the exact line shapes are more asymmetric. ͓S0163-1829͑98͒04717-1͔