Chaetomorpha is a common and widespread green seaweed genus in the order Cladophorales, characterized by unbranched filaments.Its taxonomy has been a matter of much confusion because of few morphological characters and extensive morphological plasticity.In this study, the sample of a green tide forming alga, Chaetomorpha valida (Hooker et Harvey) Kützing, was collected from aquaculture ponds in Rongcheng, P.R.China.Phylogenetic analyses of 18S rRNA gene placed C validaallied with Chaetomorpha moniligera with high support; ITS region sequences analyses demonstrated that the species was sister to the microfilamentous epiphyte Chaetomorpha norvegica, from which it was morphologically clearly distinct.Based on the unialgal culture, developmental morphological characters of C.valida were observed microscopically.