When a population goes through a small bottleneck, the genetic variability of the population is expected to decline rapidly but, as soon as population size becomes large, it starts to increase owing to new mutations. This problem is studied mathematically, and the results obtained indicate that the amount of reduction in verage heterozygosity per locus depends not only on the 'size of bottleneck' but also on the rate of population growth. If population size increases rapdily after going through a bottleneck the reduction in average heterozygosity is rather small even if bottleneck size is extremely small. On the other hand, the loss in the average number of alleles per locus is profoundly affected by bottleneck size but not so much by the rate of population growth. This difference occurs mainly because random genetic drift eliminates many low frequency alleles. However, the average number of alleles per locus increase faster than the average heterozygosity when population size is restored. Application of the theory developed to the Bogota population of Drosophila pseudoobscura supports Prakash's postulate that this population has grown very rapdily, starting with a few migrants from a Central or North American population.