The seasonal pattern of evapotranspiration (expressed as latent heat flux Qe) for a 28- to 30-m-tall tropical transitional (ecotonal) forest was quantified over an annual cycle using eddy covariance measurement and micrometeorological estimation techniques. The study was conducted near the city of Sinop, in northern Mato Grosso, Brazil, which is located within the ecotone of tropical rain forest and savanna (cerrado). Although the majority of net radiation (Q*) was consumed by Qe (50--90%), seasonal variations in Qe were large and positively correlated with precipitation. Total daily Qe for the dry season (June--September) was on average 6.0 MJ m-2 d-1, while daily Qe for the transition (October--November and April--May) and wet (December--March) season periods were 7.5 and 10.0 MJ m-2 d-1, respectively. The seasonal variation in midday (0900--1500 LT) surface conductance (gs) was also positively correlated with precipitation. Analysis of the decoupling factor (Ω) indicated that the forest was strongly coupled to the atmosphere (Ω = 0.1--0.3) over the dry season and transition periods, suggesting that Qe was under relatively strong stomatal control. Although rainfall during the study period was above the long-term (30-year) average, our results indicate that the seasonal dynamics of Qe for the tropical transitional forest were more comparable to tropical savanna than to rain forest.