Chromosomal polymorphism can be maintained by means of genic dominance, genic overdominance, recombination effects and karyotype selection. Experiments were carried out which allow us to distinguish among these factors as they effect egg hatch and larva-to-adult survival in D. subobscura. A strong karotype selection was observed. A small but highly significant recombination effect was also observed despite the fact that the material used in the experiment consisted of inbred, related strains and was therefore less likely to show such an effect than strains randomly chosen from a population. On the other hand, these strains, being inbred, were more likely to show genic dominance or genic overdominance than randomly picked samples. However neither genic dominance nor genic overdominance was detected.