Expression and transmission of foreign genes in progeny derived from transformed organisms will affect breeding procedures and efficacy, and thus the rapidity of commercialization. CryIA-transformed cotton lines from Monsanto, MON81 (cryIA(b)) and MON249 (cryIA(c)) were crossed with wild type and previously derived isolines of three cultivars having one or two host plant resistance traits - nectariless, glabrous, glandless, and high terpenoid content (high glanding). ELISAs were used to qualitatively and quantitatively assess presence and expressivity of the cryIA genes in terminal leaf tissue. Segregation analysis aided by F₃-progeny tests demonstrated that cryIA(b) segregated normally and exhibited modest dosage effects (homozygote > hemizygote). CryIA(c), however, segregated abnormally (deficiency of homozygotes) and exhibited no significant dosage effect. Expression of cryIA(b) averaged nearly twice that of cryIA(c). At the F₂ generation, cryIA protein content was significantly affected by variation among the genetic backgrounds (cultivars) and among subpopulations (families derived from isolines, within cultivars), but not by interactions between the inserts and the genetic backgrounds. F₄ lines were derived from F₂ plants selected for homozygosity of natural insect-resistance trait gene(s) and the cryIA(b) gene, and for high cryIA protein content. A two-location RCBD experiment revealed marked environmental effects on cryIA(b) expression in F(2:4) lines, both between and within locations; correlations between F₂ and F(2:4) cryIA levels differed according to the site of F(2:4) evaluation (0.81 versus 0.18). CyrIA protein content in F(2:4) lines was not significantly affected by genotypic differences, presumably because all were inbred descendants of F₂ homozygotes having the highest cryIA expression in the respective F₂ family. Overall results indicated that levels of cryIA production in cryIA-homozygous plants can vary widely, and are subject to effects of dosage, environment, and effects of insertion, somaclonal variation and/or genetic background. Selection among crosses and within segregating families may elevate cryIA expression, but progeny testing and recurrent testing of cryIA protein contents may be needed during the breeding process.