This study reports relationships between general academic self-concept and achievement in grade 3 and grade 5. Gender-specific effects were investigated using a longitudinal, two-cycle, 3-year autoregressive cross-lagged panel design in a large, representative sample of Polish primary school pupils (N = 4,226). Analysis revealed (a) reciprocal relations between general academic self-concept and achievement over time but the influence of prior achievement on self-concept was stronger; (b) on average, levels of both constructs declined over time; (c) gender differences were not observed in longitudinal relations (i.e. cross-lagged, autoregressive and intra-wave correlations); (d) girls demonstrated higher mean levels of academic achievement at both grades; and (e) average level of general academic self-concept was not gender differentiated in grade 3 but decreased more for girls. These results are discussed in the light of the theoretical and practical implications.