AbstractWe have investigated the presence and localization of an α‐spectrinlike protein and its potential role in the morphological transformation of sea urchin coelomocytes. In immunofluorescence images there is a diffuse fluorescence throughout the petaloid cytoplasm, indicating a random distribution of the spectrinlike protein prior to the transformation. As these cells form filopodia, there is a coincident appearance of a spectrinlike protein, as seen in fluorescent images, at the site of filopodial initiation. As the filopodia continue to form and lengthen, the spectrin localization parallels their development. There is a single polypeptide observed on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) gels of whole coelomocyte lysates that cross‐reacts with the anti‐α‐spectrin immunogen and comigrates with it at 240 kilodaltons.