微丝是由肌动蛋白（Actin）组成的直径约为7nm的纤维结构。肌动蛋白单体（全称为“球状肌动蛋白”，简称“G肌动蛋白”）表面上有一个ATP结合位点。肌动蛋白单体可一个接一个连成一串肌动蛋白链，而微丝则由两串这样的肌动蛋白链互相缠绕扭曲成而成。微丝这种肌动蛋白多聚体又被称为“纤维形肌动蛋白”。

微丝（microfilament）也普遍存在于所有真核细胞中，是一个实心状的纤维，一般细胞中含量约占细胞内总蛋白质的1%-2%，但在活动较强的细胞中可占20%-30%。微丝的主要化学成分是肌动蛋白（actin）和肌球蛋白（myosin），如同微管蛋白，肌动蛋白的基因组成一个超家族并有多种结构极为相似的组成。在肌细胞中至少存在4种不同的肌动蛋白。

Microfilaments, often better known as actin filaments, are the thinnest filaments of the cytoskeleton, a structure found in the cytoplasm of eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. Microfilaments are highly versatile, functioning in cytokinesis, amoeboid movement, and changes in cell shape. In inducing this cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.