An existing free-water-surface constructed wetland system at the Auburn University Poultry Science Unit was used to evaluate the effect of plant fill ratio on water temperature. Each wetland consisted of two cells in series. One series was operated with an approximate 10% fill of Sagittaria lancifolia (duck-potato). A second series contained Phragmites australis (common reed) and Scirpus spp. (bulrush) with an approximate 5% fill of plants. A third series was unvegetated and acted as a control. Water temperature was measured using thermographs placed at the midpoint of each cell with temperature readings taken hourly from July 1995 until June 1996. Water temperature was compared between each cell by using paired t-tests for the hourly temperature data. The unvegetated cells had significantly higher temperatures than the vegetated cells year round. The approximate 10% fill ratio series had significantly higher temperatures than the 5% fill ratio during the winter months. The unvegetated cells were significantly warmer than the vegetated cells for 75 of 80 instances. The unvegetated cells also exhibited greater daily variation in temperature than did the vegetated cells.