Microfilaments are typically smaller than the microtubules. These are thin, cylindrical, elongated struc­tures ranging from 40-80 Å in diameter formed by joining together the mol­ecules of actin, a protein. Hence, these are also called actin filaments as found in amoeba, yeast, skeletal and cardiac muscle cells. Contraction of muscle cells results from interactions be­tween the actin filaments and the larger myosin filaments.

Microfilaments form an extensive net­work in the cytoplasm, some are asso­ciated with the plasmamembrane to­wards inner side and others extend in the cytoplasmic core of microvilli. These are involved in changes in cell shape during development and motil­ity and in protoplasmic streaming in plant cells. Actin molecules are also found to be present in close association with the spindle in dividing cells. Ac­tin-containing microfilaments are also associated with the cleavage furrow following mitosis.