A study was conducted to determine, through ecological observations and experimental manipulation, whether the marine angiosperm Thalassia testudinum Koening 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: (1) Thalassia bloomed in St. Andrew Bay, which extended its previously described flowering range considerably northward; (2) flowering was seasonal and limited in many cases to discrete areas within a grass flat; and (3) 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: (1) vegetative growth in Thalassia was highly responsive to photoperiod and was favored by intermediate daylengths; (2) flowering was also favored by intermed...