To analyse the relationship between time and the allelism of lethal genes (second and third chromosomes together as a unit), wild males of Drosophila melanogaster were captured at different times in a given point of a Korean locality and separated into two experimental groups with month-interval and year-interval in time. Each of the two groups contained five of time series populations. The average frequencies of files carrying lethal genes were 0.8407 in the month-interval populations and 0.8202 in the year-interval populations. The rates of allelism of lethal were 0.0475 in the month-intervals and 0.0184 in the year-intervals. In both cases, it showed that the slope of the allelism of lethals decreased exponentially with time interval far apart. The average of effective sizes in the present D. melanogaster population were estimated to approximately be 1,000 in the month-intervals and 1,500 in the year-intervals.