2. A new high-mosaic strain gives endosperm mosaics with a frequency higher than that ordinarily found in heavily X-rayed ears. The various endosperm loci show differing frequencies of loss corresponding at least roughly to their relative frequencies in comnon maize. The high frequency of chromosomal aberrations is limited to the early divisions in endosperm development, the proportion of small sectors being hardly more than normal. The factor responsible for this effect is transmitted through both male and female gametes. The chromosomes derived from both the male and the female parent are affected in endosperms which have received this factor from either parent. In an F₂ progeny segregating for an unknown yellow seedling factor and for the high-mosaic factor, seedlings sectorial for the yellow seedling character were common in the progenies with high mosaic frequency. Plants heterozygous or homozygous for the high-mosaic factor are normal in development and have normally fertile pollen and ears.