During the summer of 1948 I prepared for this Journal an annotated list of the 75 species of vascular plants which had become established on the causeway constructed in the Straits of Mackinac southwest of St. Ignace (Mackinac County) by the Michigan State Highway Department. Completed and abandoned in the spring of 1944, this pile of limestone blocks extends southerly into the Straits for almost a mile, with a roadway on top the width of a four-lane highway. Anothe four years having elapsed, on July 12 and September 2, 1952, I again studied the causeway flora. Brief visits were also made on July 27 and August 6, 1953. Since the proposed bridge across the Straits is again under consideration, it is uncertain how much longer there will be opportunity to study the establishment of vegetation on the deserted causeway before it is finally put to use as the north approach of a bridge.