The development of insect resistance to the B.t. endotoxin is a concern associated with the widespread adoption of B.t. cottons. Laboratory trials were conducted in 1996 to determine if the ability of the bollworm, Helicoverpa zea (Boddie), to establish and complete larval development in B.t. cotton was due to adaptation to the endotoxin in the field population. In addition, the adaptation to B.t. endotoxins that occurs from selection of single generations in the field was measured in the laboratory to determine if and how rapidly the onset of such resistance may occur. Results with field-collected bollworm larvae suggested that adaptation to the B.t. endotoxin had not occurred in the area of collection but that there was potential for increased tolerance to B.t. (as a result of field exposure) in bollworm in as few as 3 generations. Additional lab trials will be conducted to determine if the basis for increased larval weights and pupal weights and decreased durations of pupation of bollworm larvae originally reared on B.t. cotton is indeed genetic or if environmental factors (i.e., maternal effects) influenced these differences.