Glabrous Halophila decipiens Ostenfeld has been collected from four estuaries in southwestern Australia. The plants form pure meadows or grow with Halophila ovalis (R.Br.) Hook. f. and/or Zostera mucronata den Hartog on silty sand substratum, in channels near the mouths of the Swan and Blackwood Rivers, and small patches of this species also occur adjacent to Z. mucronata and H. ovalis meadows near the mouth of Oyster Harbour in Albany. Preliminary field observations at Hardy Inlet of the Blackwood River indicate that the seeds germinate in austral summer, the plants grow through summer and set fruit and seed in autumn. Total dry weight of biomass was 92.8 g m(-2) at the time of maximum biomass in April, there were 6700 fruits m(-2) and 26.4 seeds per fruit, resulting in an estimated 176 880 seeds m(-2) in the H. decipiens meadow in Hardy Inlet.