The yellow perch, Perca flavescens (Mitchill), of Costello Lake, Algonquin Park, Ontario, were sampled regularly from June to September, 1937, by means of minnow traps. From this 89‐acre lake about 750 individuals were taken for an analysis of growth and feeding habits. The period of maximum growth changed progressively from July in age‐group I to August for fish in age‐group III. Seasonal variations in the volumes of stomach contents were directly correlated with seasonal changes in the growth increments. This correlation was so well marked that food is believed to be the primary factor controlling growth in this perch population.