The duration of the reproductive phase of development is critical to soybean yield determination. The E-gene series of loci in soybean are reported to control both flowering and maturity through a photoperiod-mediated response. The objective of the current study was to determine the role of photoperiod and E-genes on the duration of the reproductive phase. The first experiment involved seven E-gene near-isogenic line (NIL) planted to synchronize flowering. The main plots were two photoperiod treatments; i) ambient day length, and ii) an ambient plus three hour incandescent day length extension, imposed post-flowering. The second experiment had two planting dates and 15 E-gene NILs. Phenology and growing degree day data were collected. The duration of the reproductive phase was related to the allelic composition of the E-gene loci. The NILs with dominant alleles on three or more loci, had a longer reproductive phase than those with recessive alleles, when grown under identical post-flowering photoperiod and temperature. Dominant alleles at the E-gene loci increase the duration of the reproductive phase under photoperiod extension. Photoperiod extension, either artificially or by early planting, significantly increased the duration of the reproductive phase in NILs with dominant alleles at one or several of the E-gene loci. In the planting date experiment, there was a linear relationship between the post-flowering photoperiod experienced and the duration of the reproductive phase of photoperiod-sensitive NILs. It is concluded that the duration of the reproductive phase is controlled by both E-genes and post-flowering photoperiod in soybean.