The attached filamentous green algae Ulothrix zonata and Cladophora glomerata are important members of the periphyton community in the rocky littoral zone of the Laurentian Great Lakes. When these algae occur together, Ulothrix occupies the splash zone and Cladophora grows in deeper water. The two algae show definite seasonal patterns of abundance as well as distinct geographic distributions within the Great Lakes basin. The authors examined the role of light, temperature, and phosphorus supply in regulating the geographic, seasonal, and spatial distribution of these two filamentous algae. Phosphorus availability plays a major role in determining the geographic distribution of attached algae in the Great Lakes. Light intensity, acting through the balance between photosynthesis and respiration, appears to have a significant effect upon the pattern of vertical zonation. Optimum photosynthesis in Ulothrix occurs at high light levels than the optimum for Cladophora. Seasonal patterns of abundance in Ulothrix and Cladophora are consistent with their respective temperature optima for photosynthesis.