Seasonal measurements of light- and temperature-dependence of photosynthesis of the nuisance alga Cladophora glomerata (L.) Kutz were determined under laboratory conditions with algal samples collected from lower Green Bay, Lake Michigan. Photosynthesis rates were determined using oxygen difference in water circulated around the speciments. Photosynthesis reached light saturation at about 790 μmol photons m−2 S−1 (PAR) and had a temperature optimum between 28 and 31°c; maximum rates were 25–50 mg O2 g dwt −1 h-1. These parameters varied seasonally with water temperatures. There was no evidence for a decline in photosynthetic rate with increasing temperature over the ranges used in this study. It is concluded that, in this instance, such an effect is not responsible for the mid-season decline of this species. Other effects, such as those of respiration, photorespiration and phenology may, therefore, be important.