To analyze genetic diversity and phylogeny of Cracilariaceae (Cracilariales, Rhodophyta). We compared ITS (including 5. 8S rDNA) sequences of twenty three populations of five Gracilaria and Gracilariopsis species (Cracilariaceae, Rhodophyta) , with the ITS sequences of 16 Gracilariaceaen species downloaded from Gen Bank database. The result showed that the length of ITS sequences of Gracilari-aceae varies from 893 to 1 508 bp. The interspecific genetic distance ranges from 0. 041 to 0. 600; the in-traspeciflc genetic distance ranges from 0. 000 to 0. 012, the former is much higher than the latter. Two main lineages of Gracilariaceae were from; Gracilaria Hydropuntia and Gracilariopsis/Gracilariophila. 5. 8S sequences of Gracilariaceae has low variation among species and among populations, but the distinct sites were found among the genera and can be used on genus level or above. There are 9 mutant sites that can be used to distinguish the populations of G. vermkulophylla from China, Russia and USA.%测定了江蓠属Gracilaria和龙须菜属Gracilariopsis5个物种的23个群体的ITS(含5.8S rDNA)序列,并结合GenBank数据库中现有江蓠科Gracilariaceae的16个物种的ITS序列进行分析,在不同分类阶元中探讨了序列变异和和系统进化关系.江蓠科海藻ITS序列长度在893～1508 bp之间,种间遗传距离在0.041 ～0.600之间,种内遗传距离在0.000～0.012之间,其种间遗传距离均大于种内遗传距离；ITS系统发育聚类结果显示江蓠科分为两大分支,分别是江蓠属/Hydropuntia分支、龙须菜属/蓠生藻属Gracilariophila分支；江蓠科海藻5.8S序列种内种间变异很小,但存在稳定的属间区分位点,可用于属以上水平的分类鉴定；中国、美国和俄罗斯三地的真江蓠群体的ITS序列存在9个稳定的变异位点,可以将不同地理群体区分开.
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