Crop density (CD) of 'Golden Delicious' trees on three rootstocks at six locations was manipulated for four seasons to provide a range of CDs. Cumulative growing degree days (GDDs) were calculated for each season at each location. Data were subjected to multiple regression analysis to estimate average fruit weight (FW). The best model contained rootstock as an indicator variable, and the regressor variables included CD and GDD for the first 60 days after full bloom. The interaction of rootstock×CD×GGD2 was significant, so models were developed for each rootstock. FW was negatively related to CD. For M.26 and M.9, FW peaked at 675 GDD, but for G.16 FW peaked at 700 GDD. FW increased at a greater rate with increasing GDD for heavycropped trees than for light-cropped trees. For all rootstocks, FW was most affected by CD at low values of GDD.