Polygonization of aged sea ice is described and is related to internal stress. The subboundaries of the polygonization structure are accounted for by dislocation phenomena and by the freezing of brine in tapering internal fractures. Movement of subboundaries in ice under stress is illustrated. It is suggested that the very much smaller microstructure found in ice frozen from "fresh" water may have the same origin as polygonization in sea ice. Internal fracturing provides avenues for brine migration and is thought to be a significant factor in natural sea-ice purification. Polygonization structures may significantly affect the strength of sea ice.