alpha-Smooth muscle actin is the isoform of actin restricted to vascular smooth muscle, pericytes, myofibroblasts and, certain other cells that are of myoid origin. We investigated the distribution of alpha-smooth muscle actin and nonmuscle specific filamentous actin in the human aqueous outflow system by immunohistochemical methods. Filamentous actin was observed in all cellular constituents of the outflow pathway, while distribution of alpha-smooth muscle actin was restricted to the ciliary muscle, to specific cells throughout the trabecular meshwork, and to cells adjacent to the outer wall and the collector channels. The ciliary muscle extended deep into the corneoscleral meshwork, far anterior to the scleral spur. These findings agree with our previous study localizing the distribution of smooth muscle myosin in the human aqueous outflow pathway. Although functionality of the immunoreactive cells needs to be demonstrated, our data show that a potentially contractile apparatus exists in a subpopulation of trabecular meshwork cells and in certain cells of the more distal components of the outflow system.