One who crosses the Straits of Mackinac can hardly fail to notice a long structure like a breakwater extending from the southern shore of the upper peninsula of Michigan over a mile west of the ferry route. The United States Lake Survey concisely describes the situation: "A causeway has been constructed by the State of Michigan in a southerly direction from the south side of Graham Point about 1 mile west of its extremity; it extends about 4,000 feet from shore to the 12-foot contour." (See figures 1 and 2.) The causeway was built by the Michigan State Highway Department during investigations concerning the feasibility of constructing a bridge across the Straits of Mackinac. The causeway was completed in the spring of 1944, and this idle pile of large limestone blocks now extends almost a mile into the Straits and is approximately the width of a four-lane highway. The top is filled in with gravel and crushed stone and leveled so as to permit a car to travel the entire length. There is a short bridge near the north (shore) end of the causeway. The State Highway Department informs me (personal communication) that the fill on the top of the causeway was all obtained from immediately north of the site, from the great excavation which provided some of the limestone blocks themselves. No grass seed was sown on the causeway. I t may then be assumed that all plants currently found growing there were not brought in from elsewhere in the state during the actual construction of the mole. On July 2 and August 20, 1948, I collected plants on the causeway in order to determine and record just what species have become established since the project was completed and abandoned in 1944. In order to give a definite limit to the collecting area, I collected only south of the bridge near the north end. With the exception of a few muddy spots where rain water accumulates, the surface of the causeway is poor soil-mostly loose gravel, sand, and disintegrating limestone. Occasionally fishermen and curious tourists go out on the causeway, and their visits would account for the introduction of some of the plants. Another portion of the flora may be attributed to the Herring Gulls which frequent the southern end.