For wild oats, persistence characters such as seed production and seed dormancy may be affected by genetic variation and several environmental factors during the development of the parental plant. However, the effect of varying photoperiods on such characters is unclear. Previous studies have concentrated on natural populations and have not studied the genetic variability within the population. Consequently, a study was conducted to examine how photoperiod (10, 12, 14, 16, and 18 h at 20 C) influenced the persistence of different members of a synthetic population of wild oats. This synthetic population consisted of several isogenic lines with differing degrees of seed dormancy that originated from one region in North America. All lines were photoperiod sensitive, quantitative long day plants, with an increase in time to maturity for all lines (about 78 to 213 d) when photoperiod was reduced from 18 to 10 h. The lines within this synthetic population (with the exception of one, M73) showed a similar degree of photoperiod sensitivity. This may be expected, as the lines are from one region with the same photoperiod environment. It could be concluded that such a photoperiod sensitivity is a beneficial character that has allowed persistence of these lines in this region. When photoperiod was reduced from 18 to 10 h, plants produced fewer seeds (about 141 to 61 primary seeds per plant) with a higher degree of seed dormancy (about 88 to 54% germination in 10−4 M gibberellic acid; GA3). The reduced seed production under the shortest photoperiod (10 h) was due to poor panicle exsertion, which resulted in poor development of basal florets. There was no consistent relationship between photoperiod and other plant characters such as tiller production, caryopsis weight, and water content. However, significant relationships were found between seed dormancy characters and other plant characters such as maturity time, caryopsis weight, and water content under certain photoperiod treatments.