A phytosociological study of weeds carried out at Ajmer in the semi-arid zone of India revealed the presence of Dactyloctenium-Cynodon-Chloris community during the monsoon season and Cynodon-Tribulus-Tephrosia community of weeds during the summer season, growing on coarse and sandy soils and in harmony with all biotic factors. The communities experience high summer and low winter temperatures with rainfall occurring only during the monsoon season (July to September). Other dominant weeds included Achyranthes aspera L., Amaranthus gracilis Desf., Tephrosia purpurea (L.) Pers., and Trianthema portulacastrum L. during the monsoon season, and Boerhavia diffusa L., Indigofera linifolia (L.) Retz., and Rhus mysurensis Heyne ex Wight. are dormant during the summer season.