In vitro responsiveness of microshoots derived from three genetically related and different age Eucalyptus urophylla 9 Eucalyptus grandis clones kept cultivated by monthly subcultures was assessed on two rooting media in relation to the time spent in culture. Significant differences of rooting capacity were found between the two 22-yearold half sibling genotypes 147 and 149 according to the concentration of BA added to the media and also to light intensity. The contrast was even more salient with the 1- year-old clone S, which displayed the poorest rooting ability despite being full sibling with clone 147 and much younger. The various criteria observed, i.e. rooting rate, number of roots produced, root length, microshoot height and shoot tip necrosis varied greatly and differently for the three origins according to the successive time replicates of the same rooting protocols, with rooting rates of more than 80% for clone 147 at certain dates. These results were discussed, suggesting the influence of endogenous rhythms as the most rationale cause of the time-related fluctuations of responses observed in steady in vitro environment, notwithstanding possible interferences of non-optimal tissue culture conditions. Such unexpected variations of rootability between closely related genotypes, and the notable interactions pointed out between the time spent in culture and the rooting conditions deserve to be taken into consideration before stating definitive judgment on the rooting capacities of a given genotype for arborescent species.