The protein quality of maize (Zea mays L.) can be improved by replacing normal Opaque2alleles with nonfunctional recessive alleles (o2). Homozygous o2 kernels have increased levels of lysine and tryptophan. Unfortunately, the associated soft texture of the o2 kernels causes poor yield and susceptibility to diseases and insects. Breeding has resulted in the development of o2 genotypes with improved endosperm hardness; such genotypes are referred to as quality protein maize (QPM). Quality protein maize germplasm is utilized in breeding programs worldwide and has been competitive in yield trials. To understand the genetics of endosperm modification, a population of 146 recombinant inbred lines (S₅ to S₇) derived from a cross between the o2 inbred B73o2 and the QPM inbred CML161 was evaluated in two Texas locations from 2004 to 2006. The endosperm traits texture, opacity, and vitreousness were highly affected by inbred line genotype, were highly correlated with each other, and exhibited high broad-sense heritability. Relative content of the essential amino acids lysine, tryptophan, and methionine were also highly affected by the inbred line genotype, and exhibited high broad-sense heritability. Negative correlation was observed between endosperm texture traits and amino acid contents. Favorable responses to selection can be expected for both endosperm texture modification and relative content of the essential amino acids if they are efficiently monitored.