The aim of this study was to examine the role of pikeperch (Sander lucioperca L., 1758) in structuring the seasonal food web of Lake Egirdir using carbon (delta C-13) and nitrogen (delta N-15) stable isotopes. The samples for stable isotopes were collected during the spring and autumn seasons. Sander lucioperca, as expected, was determined as a top predator with a substantially higher trophic position during both seasons (3.96, spring, delta N-15 (mean +/- SE = 13.09 +/- 0.17) and delta C-13 (mean +/- SE = -17.23 +/- 0.19) (4.10, autumn, delta N-15 (mean +/- SE = 14.14 +/- 0.19) and delta C-13 (mean +/- SE = -18.15 +/- 0.02). Three different models (IsoSource, SIAR, and Bugalho et al. (2008)) were used to determine food sources that most contributed to the food of S. lucioperca. During spring, the fishes and invertebrates, Potamon potomios and Pontastacus leptodactylus, contributed most to the food of S. lucioperca, a result obtained by the 3 models. According to the IsoSource model, among the fishes, these included Vimba vimba, Capoeta pestai, Cyprinus carpio, and S. lucioperca (cannibalism); however, according to Bugalho et al. (2008) and SIAR, Vimba vimba, Cara ssius gibeleo, and Seminemacheilus ispartensis were assimilated by S. lucioperca in greater proportion during spring. During autumn, the contribution of fishes to the S. lucioperca food source increased substantially with percentages noted as 83% and 90%, a result obtained by the IsoSource and Bugalho et al. (2008) models, respectively. The SIAR model, on the other hand, identified fishes, fish eggs, and invertebrates with mean contributions of 35%, 30%, and 16%, respectively. Invertebrates were also an important food source for S. lucioperca during this season according to Bugalho et al. (2008). In conclusion, this study showed that S. lucioperca shifted its diet from invertebrates, including crayfish, during spring, to fishes during autumn, a result also supported by the increase in the trophic position of S. lucioperca from spring to the autumn. This study is the first to report results on the food content of S. lucioperca revealed using stable isotope methods in Turkey.