1. The viabilities of homozygotes for second chromosomes extracted from a wild population of D. melanogaster in one locality in Israel were investigated in six consecutive samples collected during two years. 2. The incidence of lethal chromosomes in these six samples fluctuated about a value of 32.16 ± 1.34% and the concentration of lethal and semilethal chromosomes combined amounted to 39.12 ± 1.40%. 3. The six samples did not differ significantly among one another in the sizes of the various viability classes. Thus, although the collection data indicate a population bottle neck occurring during the rainy winter months, a corresponding reduction in the concentration of lethal genes, as recorded by Dubinin for Russian populations of the fly, could not be observed. 4. The concentration of deleterious recessive genes in the present six samples showed good agreement with the data of a previously investigated population from Jerusalem. 5. Among the visible mutations extracted from the 1,222 chromosomes of the present samples no evidence of duplicity due to inbreeding was discovered. 6. The average viability of the 'normal' homozygote appeared to be consistently larger (30.20%) in the population from the Judean Hills than in the sample from Jerusalem (27.98%). 7. There was a highly significant excess of females among the unmarked flies recovered in the test generation. 8. An inversion very nearly homologous with In (2L) Cy was found in the present population. 9. The frequency of allelism amounted to 0.335 ± 0.126% among the lethals of the autumn sample of 1952 and to 0.830 ± 0.249% in the sample of the following spring. This difference very nearly reaches the 0.05% probability level and might therefore be interpreted as an indication of a reduction in the effective size of the breeding population during the rainy season. 10. It is shown that some of the data obtained by various investigators do not substantiate an inverse relationship between lethal concentration and allelism. 11. It is suggested that lethals on the second chromosome of D. melanogaster and on its homologues in some other Drosophila species frequently tend to fluctuate around an optimum concentration near 30 percent.