The opaque endosperm2 (o2) gene that alters protein composition of maize (Zea mays L.) endosperm also influences starch digestibility for ruminants. The softer, less-dense kernel texture of o2 grain directly improves starch availability, but it also adversely affects agronomic performance. Breeding efforts were initiated to improve yield and kernel characteristics of o2 genotypes as part of CIMMYT's Quality Protein Maize project. The objectives of our study were to determine whether o2 digestibility is maintained in modified o2 (mo2) and to detect major QTLs involved in control of DM digestibility (DMD). 140 recombinant inbred lines (RILs) from B73 o2/ o2 x CML161 mo2/ mo2 were planted at two locations in Texas. 1.5-g kernel samples ground through 6-mm screen were placed in 5x5 cm 50mm-pore-size dacron bags. These were evaluated for 14-h DMD using rumen-cannulated Holstein cows. Significant positive correlations between kernel modification and 14 –h DMD suggest that kernel modification reduced DMD. Composite interval mapping was run per location and combined data. Significant 14-h QTLs were detected on chromosomes 4, 7, 9, 10 (2005), 3, 4, 5, 9 (2006) and 4, 5, 7, 9, 10 (Combined). 0-h QTLs were detected on chromosomes 4, 5, 7 (2005), 3, 6, 7 (2006) and 3, 5, 7 (Combined). QTLs for “difference” (14-h minus 0-h) were detected on chromosomes 5 (2005), 6 (2005, Combined), 9 (2006, Combined). 14-h QTL on chromosome 4 was detected across analyses. mo2 alleles reduced DMD while o2 alleles increased DMD. 0-h and 14-h QTLs on chromosomes 4 and 7 and “difference” QTLs on chromosome 6 occupy same position as modifier gene clusters detected previously by others. “Difference” QTLs on chromosomes 5 and 9 occupy unique positions and are likely to be closely associated with true ruminal DMD and could be candidates for improvement of mo2 DMD.