A study was carried out to investigate and compare the population structure, reproductive biologyrnand chromosome cytology of two endemic Aloe species, A. pulcherrima (with rare occurence)rnand A. debrcma (common) on naturally occurring populations around Debre Libanos in centralrnEthiopia from May 1997 to February 1999.rnThe purpose of the study was to find out the causes for the rare occurrence of A. pulcherrima inrncomparison to the relative abundance of the sympatric species, A. debrcma with the aim ofrndeveloping appropriate conservation strategies to conserve these endemic plants.rnSix plots of (5 x 20 m2) were established at the study site; three of these include the A.rnpulcherrima and the other three include A. debrcma populations. In each plot, genets and rametsrnwere sorted out and the population structure was described by clone size and rosette diameter ofrnramets. Size of ramet relationship to flowering and size of ramet and flowering relationship torngrowth rate were studied. In addition, population flux (dynamics) at genet and ramet level wasrncalculated and compared for the two species.rnReproductive success and dispersal efficiency in the two species were estimated and comparedrnbased on the extent of vegetative propagation; production of flowers, fruits, seeds and also byrnpollinator availability and percentage pollen viability in the populations studied.rnChromosome cytological studies, i.e., number, pairing and segregation of meiotic chromosomesrnwere conducted in the laboratory from flower buds at appropriate stages of development andrncomparison made.rnEnvironmental factors which influence the development and distribution of the Aloes such asrnsoil, climate, vegetation, herbivory and human oriented activities associated in the study site werernobtained and analyzed. The results of the study revealed that A. pulcherrima has very old stagnant â€œ senileâ€ populationrnwith no juvenilization (no recruitment from seed) but entirely depend on vegetative propagationrnand spread in spite of good seed set, satisfactory pollinatory activity (Sun birds and Bees) andrnhigh percentage pollen viability. In addition, it is attacked by a rust fungus parasite (Uromycesrnaloes) and heavily grazed by goats and baboons. A. debrana on the otherhand has a more viablernpopulation with adequate recruitment both from vegetative propagation and sexual reproduction.rnMoreover, the production of flowers, fruits and seeds were found to be greater than in A.rnpulcherrima. As in A. pulcherrima, it has satisfactory pollinatory activity (sun birds and bees)rnand high percentage pollen viability. A. debrana appears to be â€œ bufferedâ€ from rust fungus attack,rnless often eaten by goats and baboons, and is expanding. The two species also differ in theirrnflower phenology; A. pulcherrima flowering from June to August and A. debrana flowering fromrnDecember to February. Thus, this avoids competition for pollinators and hybridization betweenrnthem. Both species also show low mortality at both ramet and genet level.rnChromosome cytological study showed that the two species have diploid chromosome numberrn2n=14; 8 large and 6 small sized. Meiotic activities from pollen mother cells (PMCs) were alsornfound normal with 7 bivalent (711) formation at metaphase I and absence of aberrations such asrnlaggards (univalents and/or separated chromatids), bridges and fragments at anaphase/telophase Irnand II that ultimately end up in micronuclei formation. These were also evidenced by highrnpercentage pollen viability in the anthers of the species studied.rnThus with the need to protect these endemic species, appropriate conservation strategies have tornbe suggested.