We examined how mayfly growth rates and body stoichiometry respond to changing phosphorus (P) content in food. In two experiments, mayfly nymphs were given high or low quantities of food at different carbon:phosphorus (C:P) ratios and their growth was measured. Low food quantity resulted in negative growth rates in both experiments, regardless of food P content. However, under high food availability, mayfly growth was affected by the type of food eaten, with low C:P ratio food producing more rapid growth. In addition, mayfly growth increased somewhat when P-poor food was artificially enriched with inorganic P although this effect was not statistically significant. Mayfly body P content was inversely related to body size but increased in animals fed artificially P-enriched food. A model was constructed to simulate mass balance constraints on mayfly growth imposed by the relative supply of two elements (C and P) in food. The model shows that mayfly growth should be limited by food P content at moderately low C:P ratios (c. 120, by mass). Given high C:P ratios (mean c. 270, by mass) in periphyton from oligotrophic boreal lakes, our experimental and theoretical results indicate that stoichiometric constraints are important factors affecting benthic food webs in lakes from the Canadian Shield and perhaps in other systems with similarly high C:P ratios in periphyton.