When the Chilean endemic species Drosophila pavani compete during the pre—adult stage on a limited amount of food with D. melanogaster, D. simulans, and D. willistoni, developmental periods were significantly retarded, but egg—to—adult viability decreased only in the presence of D. melanogaster. In the same experiments, two of the other three species suffered no modifications in survival and developmental time, but in D. willistoni viability was reduced in the presence of D. pavani larvae. The lengthening of the developmental period of D. pavani may result in utilization of different resources, thus avoiding the direct effects of exploitation competition. The decrease in egg—to—adult viability of D. willistoni in the presence of D. pavani does not seem to be exclusively dependent on food competition. In experiments in which the culture medium was previously "conditioned" by D. pavani larvae, but with an excess of food, the viability of D. willistoni also decreased, indicating that the metabolic waste products of the first species probably interferes with the development of the second.