Infestation of soybean by Melanagromyza sojae (Zehntner) significantly reduced plant height, leaf area, dry matter accumulation, leaf moisture content, number of branches per plant, Rhizobium nodules and nodule weight, number of pods per plant, number of seeds per pod, and seed yield. Reduction in leaf area and dry matter accumulation appeared to be major factors in reduction in seed yield in plants damaged by M. sojae. Although the M. Sojae damage was similar in plants grown under an extended daily photoperiod of 13 h: 10 min and under Taiwan’s autumn photoperiod of 12 h at planting to 10 h: 40 min at harvest, only in those plants that were grown under the extended photoperiod was there no significant reduction in dry matter accumulation and yield. This appeared to be due to the delay in maturity under extended photoperiod, which provided an additional growing period to accumulate dry matter to compensate for the damage caused by the M. Sojae infestation.