1. When ascospores of Neurospora tetrasperma were irradiated with 11 kv. X-rays, the single spore cultures obtained displayed a wide variety of mutated forms.
2. Control germinations of ascospores showed uniform behavior, ranging from 92–95 per cent germination.
3. The shape of the survival curves was found to be a function of the criterion of death. The following criteria were used: germination, growth, production of mature ascospores, and the production of normal perithecia.
4. The germination survival curve exhibited a rhythmic variation with dosage. Germination is not a significant criterion of death.
5. Half-survival dosages for growth and ascospore production were approximately 30,000 and 20,000 roentgens, respectively.
6. Multiple hit-to-kill relations were found on the basis of the quantum hit theory; no accurate analysis was possible.
7. The studies indicate that ascospore death does not result from a single well defined reaction, but rather from the integrated effects of several deleterious processes initiated by the radiation.