Edible Cladophora, known as "kai" in northern Thailand, is an economically and ecologically important green algal genus. Although Cladophora has been the focus of many systematic studies, there is an urgent need for taxonomic re-evaluation of kai. We collected 19 isolates of kai from 25 sites in northern and northeastern Thailand during the cool-dry season and during summer. Morphological observation revealed that kai comprised Aegagropila linnaei K tzing and Cladophora glomerata (L) K tzing. A. linnaei occurred only in the Golden Triangle in the northern Thai province. C. glomerata occurred commonly in all locations surveyed, and was so variable that we were able to distinguish 12 morphotypes. Phylogenetic analysis of SSU rDNA sequences corroborated our morphological identifications of A. linnaei and C. glomerata, and revealed the monophyly of all 12 C. glomerata morphotypes. However, infraspecific variation in C. glomerata SSU sequences was greater than reported variation among other species within the genus. This is the first systematic study of edible Cladophora in southeastern Asia based on morphology and molecular (SSU rDNA sequences) comparisons.