The morphological and photosynthetic activity were characterized in six genotypes of American oil palm (E. oleifera) of different origins and three of their OxG hybrids (E. oleifera × E. guineensis) with a completely randomized experimental design, three replications and 16 palms per replication. Determinations were made for: photosynthesis, internal CO2 concentration, chlorophyll content, stomatal openings and density, specific leaf area, and measurements of vegetative growth. The genotypes

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... e differentiated by growth rate, emission, area and dry weight of the leaves. The stomatal openings of all the genotypes were maximum in the morning hours, with partial closures at midday; therefore, the maximum rate of photosynthesis was between 9:00 and 12:00 hr. The "Perú" American oil palm and "hybrid 2" (Sinú-Coarí × La Mé) showed higher CO2 internal concentrations, total chlorophyll contents, light saturation points and photosynthetic rates, even with low levels of photosynthetically active radiation; characteristics that indicate a high capacity for the fixation of CO2. The photosynthetic rate showed a high positive correlation with the chlorophyll content and a negative correlation with the specific leaf area. In conclusion, the "Perú" American oil palm showed relevant characteristics for use in breeding programs as female parent of OxG hybrids, while "hybrid 2", due to its outstanding morphophysiological characteristics, is considered a genotype with good agronomic performance