Nine natural populations of subterranean clover (Trifolium subterraneum L.) coming from different eco-geographical sites of the North-East Algeria, have been studied for their chromosome number and karyotype features. The study is part of the evaluation and valorization of plant genetic resources of fodder and pastoral interest in Algeria. The results of mitosis detect two groups of populations, and reveal diversity in the number among and within populations. The Algerian populations of T.

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... rraneum are characterized by two chromosomic formulas. The first formula (2n=2x=16m) (median), more common in most of the studied populations, is in conformity with previous reports in this species. The karyotype of these populations is symmetrical for size and form. The second (2n=2x= 18m), is detected for the first time and described as a new chromosomal formula in T. subterraneum. The latter is relatively more frequent than the first one and characterizes the populations coming from high altitude areas. The karyotype (2n=2x=18m) is relatively symmetrical. At the level of the two established Karyotypes, satellites are highlighted at the first pair. A variation in the size and frequency of these satellites is observed. The species exhibits regular meiotic behaviour, confirming the presence of two basic chromosome numbers (x=8 and 9). The study also highlights the role of ecological factors (Altitude and Rainfall) of the originating environment of Algerian populations in the variation and evolution of chromosome numbers in T. subterraneum. The new cytogenetic data can be exploited in the taxonomy of the species in Algeria in order to select and develop this plant genetic resource in the agricultural field.