Summary Meiotic studies were carried out in 2 different accessions of Tribulus rajasthanensis (Zygophyllaceae), a threatened species of Rajasthan, India. This plant is being over exploited for its multipurpose utility as a fodder and medicinal uses, threatening its occurrence in natural habitat. Two accessions belonging to T. rajasthanensis collected from 5 different locations were cytologically analyzed, which showed the same gametic number of nϭ12 consistently meiotic analysis in pollen mother cells revealed predominance of ring type over rod bivalents, which had mostly terminalized chiasma, giving terminalization coefficient of 0.93 in both the accessions. Anaphase I distribution of chromosome was normal, although univalents in the form of laggards were often encountered. A significant reduction in percentage pollen stainability could be correlated to rather chromosome associations indicating heterozygosity in the constituent genome T. rajasthanensis. The role of structural/numerical chromosome change in divergence of T. ragasthanensis is discussed in detail.Key words Tribulus rajasthanensis, Meiosis, Chromosome associations, Heterozygosity, Pollenstainability.Tribulus rajasthanensis is an endemic and threatened species reported from western arid regions of Rajasthan (Bhandari 1990). It is a very common species on Massuria rocks and other sandstone hills in the vicinity of Jodhpur and Jaisalmer areas (Bhandari 1990). Only scattered plants in patches are found in Massuria rocks and Machia safari park of Jodhpur. Some of the specimens were observed in Nagaur and Bikaner regions. It grows well on rocks and hill regions and flowers immediately after the first showers. The size of flower, leaves and fruits are also bigger and hairy. It is a perennial or rarely annual, diffusely prostrate or somewhat ascending herb. The leaves are long, opposite or sometimes alternate in the lower region, and always unequal at the node. The flowers are bright yellow in color. Sepals are linear-lanceolate, acute, margins scarious, villous to outside, and minutely pubescent to inside in upper half. The petals are broadly obovate-cuneate, the ovary is bulbous-based bristly and the style is long, elongate, and cylindrical. Fruit are cocci and long, divergent spines inserted almost centrally with 20-25 unequal, smaller secondary ones crested as well as dispersed at the back and often reaching half the length of the main spines; every spine terminated by a tuberculate bristle much longer than the spine itself (Bhandari 1990).Despite its lesser abundance, very limited information about T. rajasthanensis, a threatened species is available about the economic importance in general and medicinal importance in particular, it is quite commonly used by the local people for different ailments including hyperglycemia. It is a common ingredient in auyrvedic preparations. However a detailed investigation regarding the active compounds as well as their clinical properties needs to be carried out in Tribulus rajasthanensis. Therefore in the present in...