A study was conducted to determine, through ecological observations and experimental manipulation, whether the marine angiosperm T. testudinum Koenig and Sims is responsive to photoperiod. The field observations, made in St. Andrew Bay on the northwestern Gulf Coast of Florida and in Biscayne Bay on the southern Atlantic Coast of Florida, revealed that: Thalassia bloomed in St. Andrew Bay, which extended its previously described flowering range considerably northward; flowering was seasonal and limited in many cases to discrete areas within a grass flat; and natural flowering in Thalassia was apparently influenced to some extent by photoperiod, as indicated by its seasonal nature and response to water depth and clarity. Laboratory culture of Thalassia at 6-, 12-, 18-, and 24-hr. photoperiods resulted in the following observations: vegetative growth in Thalassia was highly responsive to photoperiod and was favored by intermediate daylengths; flowering was also favored by intermediate daylengths; the flowering response of Thalassia to photoperiod appeared to be one of expression rather than induction; and there was an apparent requirement for a minimum daylength as well as a minimum nightlength. These observations led to a tentative classification of Thalassia as an "intermediate-day plant" with respect to flowering and vegetative growth.