In order to characterise the physiological phenomena which occur during acclimatisation of oil palm (Elaeis guineensis Jacq.) plantlets produced through somatic embryogenesis, a comparison of the growth and carboxylase activities of in vitro propagated plants and seedlings was carried out over a 100-day period. Growth parameters (total fresh weigh (FW), relative foliar FW and the number of expanded leaves) and biochemical characteristics (soluble protein and chlorophyll content, specific Phosphoenol pyruvate carboxylase (EC 4.1.1.31) (PEPC), Ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.38) (RubisCO) activities and relative RubisCO content) were studied. Oil palm in vitro propagated plants were found to undergo an original pattern of acclimatisation, as their PEPC/RubisCO ratio was not affected during transplanting to the greenhouse environment and remained at the same level (ca. 0.05) as was measured in in vitro growing leaves. At about D60 after sowing (or ex vitro transplanting) the main physiological characteristics (chlorophyll and soluble protein contents, and PEPC/RubisCO ratio) were similar in both seedlings and in vitro propagated plants, but growth characteristics were markedly different. Rocket immuno-electrophoresis revealed that relative RubisCO amounts were in a comparable range (ca. 230 mg/g prot.) in leaves from in vitro grown and already acclimatised in vitro propagated plants and were found to be lower than in greenhouse-cultivated adult oil palms (350 mg/g prot.).