A sample of ten trees of Quercus oocarpa Liebm, growing in a natural stand, as studied in La Carpintera protecting zone, Cartago, Costa Rica, from August 1993 to July 1994. Leafling, leaf fall, blooming and fruiting were recorded monthly for each tree. Clirnate conditions and water content of shoots and soil were also recorded. Leaf growth and deyelopment occur mainly during the dry season ( February ) and leaf fall is more frequent from January to March ( beginning and middle dry season ). These results suggest that phenological reactions depend on the interaction between decrease in rainfall and the presence of low temperatures in the late afternoon and night. Blooming is more intense during the dry season, with longer periods of daylight. Fruiting occurs mainly at the rainy season ( 60 to 90 days after the most intense period of blooming ).Se estudio el Quecus oocarpa en la zona protectora "La Carpintera". Cartago. Costa Rica. detenninándose mensualmente la brotadura de hojas. caída. de follaje. floración y fructificación. así como las condiciones climáticas, el contenido de humedad del suelo y del tejido vegetativo de las ramas. La brotadura de hojas se concentra durante el período seco (febrero). La caída de follaje ocurre de enero a marzo (a principios y mediados del período seco). Este proceso ocurre cuando decrece la precipitación y temperaturas. La floración esta asociada a las bajas precipitaciones y menor cantidad de horas de oscuridad. La fructificación se relaciona con la época de mayor precipitación (60 a 90 días después de la floración).