Actin was identified by immunoblotting with two monoclonal antibodies in 13 species of foraminifera representing four major taxonomic groups (Allogromiina, Textulariina, Rotaliina and Miliolina). Three actins of different molecular weight (MW) were detected. The MWof these actins (about 43, 45 and 46 kD) is larger than that of vertebrate skeletal actin and protistan cytoplasmic actin. Two actins of different MW were detected in Allogromiina, Textulariina and Rotaliina, while only one type was present in Miliolina. Differences between actins of different taxonomic groups in foraminifera suggest that actin might be a useful tool in the study of the molecular phylogeny of these microorganisms.