An experimental population of D. pseudoobscura was maintained for 4.5 years, to study the stability of its initial chromosomal polymorphism. It took some 17 months (20 generations) for the population to achieve its final equilibrium frequencies, and the chromosomal polymorphism was then maintained for the remainder of the experiment, a period of 3 years (46 generations). The different karyotypes of the population did not exhibit any preferences for different parts of the food cups as egg laying sites.