Despite considerable interest in how the balance of light and nutrients affects aquatic ecosystems, few experimental studies have examined the simultaneous effects of both factors on lotic primary producers. We performed repeated experiments in which three levels of light and two levels of phosphorus were applied in a factorial design to six large experimental streams. Periphyton biomass and whole-stream gross primary production were strongly and simultaneously affected by both light and phosphorus, even at subsaturating levels. Phosphorus availability appeared to partially compensate for photon scarcity, and vice versa. Chlorophyll:dry mass ratios in the periphyton increased at higher phosphorus concentrations, especially at high irradiances. Larger diatoms such as Melosira varians and Synedra acus dominated assemblage composition at high irradiances while small diatoms such as Achnanthidium minutissima dominated at low irradiances. Meridion circulare, a diatom that formed macroscopic mucus colonies, was only abundant when phosphorus concentration was low and irradiances were high, perhaps because of a high carbon requirement for mucus production. Light and phosphorus influenced periphyton stoichiometry, but the strength of this influence differed for nitrogen and phosphorus. The potential additive and interactive effects of light and nutrients on primary producers and consumers have implications for both basic and applied research.