Recent experimental studies showed that stereocilia actin cores are dynamic, with assembly and disassembly rates that match their lengths (i.e. longer stereocilia have faster assembly rates). A major goal of this work was to determine the molecular mechanisms that underlie these tightly regulated assembly rates. Computational modeling demonstrates that a combination of myosin-mediated active transport of actin-polymerization upregulators to the barbed-ends of stereocilia actin along with diffusive transport of actin-polymerization inhibitors can account for the experimentally observed relationship between length and polymerization rates. Using quantitative immunostaining, fluorescence-tagged protein expression imaging, and live video confocal microscopy, I show that two classes of unconventional myosins, myosins III and XV, transport actin-regulatory cargo to stereocilia tips and that this transport directly influences stereocilia (and other actin protrusions) lengths. Specifically, myosins IIIa and IIIb transport the actin bundling protein espin-1 to stereocilia tips, whereupon the espin-1 WH2 actin-monomer binding domain is involved in upregulating the lengths, resulting in a synergistic elongation of actin protrusions when these proteins are co-expressed. While the WH2 domain is necessary for this enhanced elongation, it is not sufficient—the bundling activity of espin-1 is also necessary. Myosin-XVa similarly elongates actin protrusions via the barbed-end directed transport of the actin bundling and capping protein eps8. It should be noted that both of these motor proteins—myosin-III & myosin-XVa—are implicated in inherited human hearing loss. Thus, these studies provide a framework by which to understand the physiopathology of these genetic diseases. From a framework by which to understand the physiopathology of these genetic diseases. From a general cell biology perspective, this works provides solid evidence that at least two barbed-end directed myosins can upregulate actin filament elongation via transport of actin-regulatory cargo to the polymerizing barbed-ends of filaments in actin protrusions.