Objective: Avian smooth muscle synaptopodin 2 nucleates and polymerizes actin under the control of Ca++-calmodulin. Our goals were to define the gross structure of the actin filaments formed in presence of synaptopodin 2 and to determine the direction of growth of such filaments. Methods: Electron microscopy of actin filaments that were formed in presence of varying concentrations of synaptopodin 2 was performed. Using electron microscopy and fluorescence measurements we changed the polymerization conditions, this is change of ionic strength and polymerization in presence and absence of barbed end blocking molecules. Results: Electron microscopy of actin filaments shows that synaptopodin 2 produced long actin filaments that became organized into thick bundles. The structure of these bundles depended on the ratio of synaptopodin 2 to actin. Most frequently the bundles were parallel and ordered. In some cases, the bundles were frayed and displayed single actin filaments. The filaments within a bundle were in-register and had either common initiation or termination points. Decoration with S1 showed that the filaments of a bundle had the same orientation. We found that synaptopodin 2 increased the critical concentration for actin polymerization in solution. In presence of barbed end blockers synapotopodin 2 induced actin polymerization did not change. Conclusions: Synaptopodin 2 appears to nucleate actin filament growth and promote growth from the pointed end growth of actin filaments. The resulting filaments tended to form tight bundles. The presence of synaptopdoin 2 in dense bodies and Z-lines suggests that synaptopodin 2 may help to cap barbed ends of filaments in these structures. The function of synaptopodin 2 must be considered along with the other actin binding proteins that define Z-lines and dense bodies.