Growth cones of chick sensory ganglion neurons in tissue culture were photographed at 60-sec intervals as they advanced over the substratum. Numbers of microspikes (or “filopodia”) were recorded together with the time and position of their appearance, their rate of elongation, their lateral movements, their lifespan, and the position and manner of their disappearance. All microspikes go through cycles of extension, lateral movement, and shortening. These are irregular and unpredictable but show systematic differences depending on where on the growth cone they occur. At the leading edge of the growth cone microspike extension occurs at highest frequency and microspike shortening occurs at the lowest frequency; when the latter occurs in this region it often involves the advance of the margin of the cell in the form of a lamellipodium. Microspike loss occurs most often at the base of the growth cone, usually by the retraction of the microspike into the cell. Calculations of the gain and loss of microspikes at different regions of the growth cone show that they undergo a net retrograde flow, the rate of which is correlated with the forward advance of the growth cone. Individual microspikes can also move backward from the growth cone onto the axon (or “neurite”), an event that occurs most often on adhesive substrata. Our observations support a direct role of microspike movement in the advance of the growth cone. The primary force for axonal elongation appears to be the contraction of microspikes pulling the leading margin of the growth cone forward. At more proximal and peripheral regions of the growth cone, microspikes undergo a retrograde sweeping motion, followed by retraction into the cell, which may also contribute to the forward movement of the growth cone. We interpret these movements as arising from a flow of actin filaments and associated proteins which are incorporated into microspikes and lamellipodia at the leading edge of the growth cone, passing backward, and being deposited into the actin-rich membrane- associated cortex of the axonal cylinder.