Somatic chromosome numbers of 2n = 20 are reported for all eight species of AustralianPosidonia: P. angustifolia, P. australis, P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsonae, andP. sinuosa. All species apparently have five larger and five smaller pairs of chromosomes. There is no evidence that speciation inPosidonia, which is relatively prolific in comparison to other seagrasses, is accompanied by change of chromosome number.