Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), one of the most conspicuous defoliators of thistle weeds, is capable of severely damaging thistle leaves; however, populations rarely reach sufficient density for effective thistle control under natural conditions. To investigate the impact of natural mortality factors on C. rubiginosa populations, life table studies were conducted between 1996 and 1998 in Kanazawa, Japan. Egg mortality, mortality in early larvae, and lost fertility contributed strongly to total generational mortality in every year studied. Egg mortality was