To determine the degree of emphasis on phonics instruction necessary for beginning readers, a study was conducted involving five pairs of four-year-old monozygotic twins. Randomly selected twins from each pair were exposed to two different levels of phonics instruction. In one group, the reduced treatment group, only one sound equivalent per letter was taught. In the full treatment group, a vowel digraph, a diphthong, and a long vowel sound were taught, in addition to the sound equivalents taught to the reduced treatment group. Analysis of the data demonstrated highly significant differences in the number of test trials that it took for the two groups to decode three phonemic sets of words. Twins taught only one sound value per letter needed fewer test trials to decode words than did their siblings whose experience included learning variable grapheme/phoneme correspondences. (Author/FL)