Pollen from maize plants genetically modified to express endotoxins from Bacillus thuringiensis has been identified as a potential hazard to the monarch butterfly, Danaus plexippus, developing in and near maize fields. We conducted two field experiments (Minnesota, USA, in 1999) to examine the effect of Bt-maize on larval survival. A two-parameter Weibull model was used to perform detailed comparative survivorship analyses. Survival on milkweed plants near Bt-maize and non-Bt-maize was similar. The larval mortality rates were lower on milkweed plants located 0, 1, and 2 m from Bt-maize compared with larvae 8 m from the maize. The cardinal direction from Bt-maize did not influence larval survival. Multiple rain events likely resulted in the relatively low Bt-maize pollen densities on milkweed leaves. We present evidence that late instar movement may bias the estimates of survivorship in field studies.