1. Approximately 20 per cent of the third chromosomes found in wild populations carry lethals.2. Less extensive data indicate a similar frequency for the second chromosome.3. On the average, about 1.3 per cent of the lethals found may be expected to be allelomorphic to any given one.4. The average frequency for any one lethal is about one-fourth of one per cent of the chromosomes of wild strains.5. The lethals, so far as studied, are completely recessive.6. Study of flies homozygous for non-lethal third chromosomes shows a considerable variation in their viability. Some are at a definite disadvantage, others apparently not. On the average they are not quite as viable as flies carrying two different third chromosomes.