Photoperiod duration has been shown to induce reproduction in angiosperms and liverworts. In general, moss gametangial production has been considered non-respondent to photoperiod, yet has been insufficiently tested. Here, we tested the response of gametangial formation to day length cues in California populations of Pohlia nutans using controlled light levels and variable day length treatments. Surprisingly, we found that gametangia induction in these P. nutans populations responded to changes in day lengths (long-day to short-day) rather than constant short- or long-day photoperiod regimes, as has been previously reported in a few moss species. We discuss our results with respect to photoperiod induction of gametangia in mosses.