In freshwater planarians the existence of at least two major actin isoforms has been demonstrated. One isoform is expressed by differentiated muscle cells and is different from vertebrate α‐sarcomeric and α‐smooth actin. The other isoform is recognized by a specific anti warm‐blooded vertebrate α‐smooth muscle actin monoclonal antibody. This latter isoform is expressed mainly in mesenchymal‐like cells ‐ generally called neoblasts ‐ activated during regeneration and growth. To date planaria is the only invertebrate where the existence of an antigen related to α‐smooth muscle actin has been demonstrated. Further studies in this field may be important to define the phylogenetic position of Platyhelminthes.