Over a five year period we compared summer temperature and dissolved oxygen (DO) at 10 sites within dense beds ofMyriophyllum spicatumL. to an unvegetated reference site in a moderately eutrophic Wisconsin lake. Average surface temperatures in the Eurasian watermilfoil beds were significantly elevated in two of the five years, while bottom temperatures were significantly depressed each year. Dissolved oxygen concentrations in surface layers of the bed were elevated by at least 1 mg L−1during roughly 30–40% of site-date visits, while DOs in the bottom layers were depressed on the average more than 50% of monitoring visits. Both depressed DOs and low DOs (< 3.0 mg L−1) in bottom layers were positively correlated with average annual air temperatures but were not directly related to variations in milfoil biomass. Dissolved oxygen and temperature appeared to be most severely impacted at shallow, densely vegetated sites close to shore.