the somatic chromosome numbers of the eleven Australian seagrass species belonging to five genera in the family Cymodoceaceae were determined.the chromosome number is Amphibolis and Thalassodendron are reported for the first time. Cymodocea and Halodule species heve the following chromosome number. Halodule has the highest chromosome number among the seagrasses and they are the largest in size with a distinct bimodal type in the family. Syringodium isoetifolium Dandy has 2n=20.