Aspects of adult host plant preference and preimaginal development of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), were investigated in spring canola, Brassica napus L., in the Mixed Grassland Ecoregion of western Canada. In spring, overwintered adults initially occurred in high densities on volunteer canola and other brassicaceous weeds. In a mixed stand of Sinapis arvensis L., Thlaspi arvense L., and Descurainia sophia (L.) Webb, weevil numbers were significantly higher on S. arvensis than the other host plant species. Regardless of host plant species, adults were found more abundantly on inflorescences than on leaves or stems. Little adult migration to spring canola occurred when crops were in the seedling and rosette stages, but migration increased significantly to a maximum in the bud and flowering stages. Most oviposition occurred when plants were still flowering but pods on lower racemes were elongating. First instars were most abundant when seeds in lower pods were beginning to enlarge. The greatest abundance of second instars occurred when seeds within the lower siliques were fully enlarged. Third instars were most abundant when seeds in lower siliques were green. Development from egg to adult required 31–58 d in spring canola. Larvae occurred more abundantly on lower regions of racemes than on pods higher up. Although C. obstrictus is primarily a pest of autumn-seeded “winter” crops of B. napus in Europe and the northwestern United States, our study demonstrated that its preimaginal development occurred rapidly and successfully within the short-season “spring” crops of B. napus grown in western North America.