Abstract: Casuarina equisetifolia, an introduced species, is a good shelter species planted along coastlines due to its enduring sandy and saline habitats.Decline was,however,observed in C. equisetifolia plantations in many areas since 1980's. Mating system and inbreeding retrogression of a plantation of C. equisetifolia in Xiamen and their roles in the decline of C. equisetifolia forests were thus researched using allozyme technique.Multilocus outcrossing rate was 0.622, lower than that expected from related species,indicating a mixed mating system,and more inbreeding caused through this introduction.Calculations from electrophoresis data showed that very a high inbreeding retrogression occurred in the C. equisetifolia plantation,indicating that inbreeding an its subsequent retrogression played an important role in the decline of C. equisetifolia plantation.Alleviating the decline through genotype species introduction was also proposed.