4. Considerable heterosis is manifest when Purdue 39 is crossed with Connecticut 30, a reduced type of P39. The P39.C30 hybrid in 1942 produced 25-30% more grain than P39. The hybrid also grow faster than either parent. The C30 type plant is recessive to P39 and the P39.C30 hybrid gives good monogenic ratios in both F2 populations and in backcrosses to C30. C30 arose in 1933 in a selfed ear of the P39-16 stock of the Crookham Company, Caldwell, Idaho. Since there was no evidence of outcrossing it is assumed that C30 is a mutation. The interesting question is whether the heterosis found last year in the P39.C30 cross was produced by the same factor causing the C30 plant to be reduced or due to other factors that may have mutated since the C30 was separated from Purdue 39. Crosses made last year may give information on this point. C30 was crossed by several different sub lines of Purdue 39 maintained in different places and quite distinct in themselves. It will be interesting to see if as much hybrid vigor is obtained when P39-16 is crossed by C30 as when other more remotely related lines are crossed. The data on hand are insufficient to justify any conclusion regarding the nature of the hybrid vigor encountered, in this intra-inbred hybrid. It could be explained by the interaction of alleles, divergent in function as suggested by East. Further study is necessary to determine whether the factors responsible for heterosis allelic or not. Whatever the explanation this phenomenon like hybrid vigor between different inbreds, may have its practical application before we understand fully the cause of the hybrid vigor. If the yield of Purdue 39 can be increased 25% or even 10% by first crossing with C30 it would seem logical for the seedsmen to use the C30-P39 hybrid in production fields wherever P39 is ordinarily used as the seed parent. Since it has been found that C30 hybrids are equal if not superior to P39 hybrids, seedsmen might well utilize the hybrid vigor of the P39-C30 hybrid in their seed fields to increase their seed yield without sacrificing in any way the quality of the finished hybrid.