SUMMARYAll normal sized spores ofNeurospora tetraspermashould contain two nuclei of opposite sex reactions at their origin. Actually only 90 per cent of these proved by experiments to be such. The other 10 per cent tested as unisexual. Three possible explanations for this discrepancy are as follows:1.If the normal sized spores testing as unisexual contain two living nuclei the sex attraction, previously exerted in the ascus, is not strong enough toalwaysbring unlike sexes together in one spore (FIG. 4, 5, 6, 7).2.If there are nuclei of only one sex after the spore germinates the nuclei of the other sex may have been killed in the process of heating the spore to bring about germination (FIG. 1, 2, 3).3.If there is only one nucleus that functions it may be that the one nucleus failed to divide when the first nucleus did and was thus crowded out and eventually died, i.e. in the processes (nuclear divisions) leading up to spore delimitations it may be, as Campbell (1937) points out forGelasinospora tetrasperma, that one or more of the nuclei failed to divide along with the others and therefore degenerated. This would leave fewer than eight nuclei to be included in the spore complement.The writer wishes to express her appreciation to Dr. B. O. Dodge, under whose supervision these experiments were conducted.