The present study was carried out at Sharkeia governorate, during cotton growing seasons of 2001 and 2003, to investigate the relation between both sowing dates and insecticidal protection periods for controlling bollworms, and their effect on cotton yield. Five experimental fields representing five sowing dates, starting from March 10 to April 30 (with 10 days intervals), were selected for this study. Seven periods of insecticidal protection applications (25, 31, 37, 43, 49, 55 and 61 days) were used per each sowing date. Five replicate areas, of 200 m2 each, were selected to estimate cotton yield for each combination between sowing dates and protection periods. Results indicated that obtained yields and other fruit structures were function of sowing dates (delay resulted in negative relation) and provided protection period (positive). Relationships between protection period (days) and physiological time (as accumulated heat units) with cotton yields and fruit structures over different sowing dates at end points were significantly positive. Predicted cotton yield, at the end of each protection period, could be obtained from the following equation: "Yield= 2177.76 – 136.7 date + 34.188 time", with P value for date = 0.0001 and for time = 0.0001, and Model R2 = 0.9342.