  We report a new measurement of A_c using data obtained by SLD in 1993-98.
This measurement uses a vertex tag technique, where the selection of a c
hemisphere is based on the reconstructed mass of the charm hadron decay vertex.
The method uses the 3D vertexing capabilities of SLD's CCD vertex detector and
the small and stable SLC beams to obtain a high c-event tagging efficiency and
purity of 28% and 82%, respectively. Charged kaons identified by the CRID
detector and the charge of the reconstructed vertex provide an efficient
quark-antiquark tag, with the analyzing power calibrated from the data. We
obtain a preliminary result of A_c = 0.603 \pm 0.028 \pm 0.023
