Mitotic spindles are elaborate, captivating structures that segregate condensed chromosomes through the coordinated activity of microtubules and force generating motor proteins. Study of mitotic spindles is therefore centered almost entirely on microtubule-based activities. In contrast, the textbook view of the mitotic actin cytoskeleton has relegated actin filaments to cortical interactions at the cell edges, far from the mitotic spindle. However, it is likely that cytoplasmic arrays of filamentous actin (F-actin) may have been overlooked due in large part to the difficulty of preserving and visualizing fine actin networks. My work characterizes a novel population of actin structures at the mitotic spindle in the intact epithelium of a vertebrate system. In Chapter 1, examination of endogenous F-actin localization revealed actin cables that spanned between spindle poles and the cortex, and reached from poles inward along the spindle body. These structures altered over the course of mitosis and live imaging experiments showed they moved dynamically with the spindle. In Chapter 2, I employed chemical inhibitors of actin polymerization, actin nucleation activity, and microtubule polymerization to determine the function of spindle F-actin. The results of these manipulations indicated regulation of actin at the spindle by one or more formin proteins, which generate new actin filaments, and further pointed to potential actin:microtubule crosstalk activity. In Chapter 3, I conducted a candidate screen of formin localization during mitosis. I identified the Xenopus formin, inverted formin 2 like (IFL2) as a regulator of spindle-associated F-actin. Knockdown of IFL2 resulted in a decrease of actin at the spindle, and overexpression or dominant negative IFL2 expression caused aberrations in both spindle F-actin structures and the spindle. Collectively, these results describe the localization of F-actin to the mitotic spindle and suggest a potential direct role for actin at the spindle during mitosis.