The electronic energy levels and charge distribution associated with kink (K) and kink-antikink (K-K\ifmmode\bar\else\textasciimacron\fi{}) defects in polyacetylene have been calculated with use of the first-principles local-density-functional theory. Finite-chain models with up to 40 carbon atoms were studied, in a self-consistent-field molecular-orbital approach. For the kink defect on trans-polyacetylene (PA), a half-filled midgap level was found, which corresponds to a neutral soliton having spin (1/2. Its formation energy was found to be 0.51 eV. Two defect levels were found for kink-antikink pair constructed on both the trans- and cis-PA chains. In the trans case, the formation energy is about 1.1 eV, which remains almost constant with increasing separation between K and K\ifmmode\bar\else\textasciimacron\fi{}. The formation energy in the cis case increases monotonically with increasing separation and ranges from 0.3 eV for two intervening C atoms to 1.5 eV for 18 C atoms between K and K\ifmmode\bar\else\textasciimacron\fi{}.