The susceptibilities of 18 field populations of the sugarcane borer, Diatraea saccharalis (F.), to two sources of Bacillus thuringiensis (Bt) Cry1Ab protein were determined using laboratory bioassays.  Fifteen of the 18 field populations were collected from seven locations across Louisiana and the other three populations were sampled from the Gulf Coast area of Texas during 2004?2006.  Neonates of D. saccharalis were exposed to a meridic diet treated with selected concentrations of Cry1Ab protein.  Larval mortality was measured at 7 days after inoculation.  Statistically significant differences in median lethal concentrations (LC50s) were detected among insect populations from different geographical locations, but the field populations remained as susceptible as a laboratory strain of D. saccharalis that had been maintained in the laboratory for >20 years without exposure to any chemical insecticides or Bt toxins.  The LC50s of Cry1Ab protein, which was extracted from DKC69-70 Bt corn hybrid, ranged from 0.03 to 0.32° ìg/g for the seven field populations collected during 2004.  The LC50 values based on bioassays with purified, trypsin-activated Cry1Ab protein from a recombinant Escherichia coli culture were 0.03?0.17° ìg/g for the 11 field populations collected during 2005?2006.  Small changes in Cry1Ab susceptibility were detected among crops, years of sampling, or locations.  All field-collected insect populations, except one, exhibited lower LC50 values than the laboratory strain.  The results of this study suggest that field populations of D. saccharalis remain generally susceptible to the Cry1Ab protein after 8 years use of transgenic Bt corn in Louisiana and the Gulf Coast area of Texas.