There are many observations which suggest that in the Drosophila genus individuals carrying diverse genetic arrangements in their chromosomes display a differential capacity to survive under crowded conditions (Birch, 1955; Lewontin, 1955; Spiess, 1958; Ayala, 1970; Budnik et al., 1971; Parsons, 1973). In the Chilean endemic species D. pavani, Budnik et al. (1971, 1972) have demonstrated that among the flies which developed under crowded conditions there are significantly higher frequencies of heterokaryotypes in respect to homokaryotypes for the inversions found in the right and left arms of the fourth chromosome. As there is a decrease of the egg-to-adult survival and a lengthening of the developmental period under these circumstances, it was inferred that the superiority of the heterokaryotypes was related to a better competitive ability. During the preadult stages, species or populations can experience two aspects of competition: (1) exploitation, that refers to the use by competing individuals of resources in short supply, mainly food and space, and (2) interference, that occurs if the resource is exploited less efficiently when two species or genotypes compete than when only one is present. In a series of experiments, Budnik and Brncic (1974, 1975) have demonstrated that the decrease in egg-to-adult viability of D. pavani under crowding conditions cannot be attribhited to food and space restrictions only. There are strong indications that the viability of the species is deeply affected also by the biotic waste products of the growing larvae. In view of the above fact, it was considered of interest to study the effect of the accumulation of the larval metabolic wastes on the survival and development of the individuals carrying different gene arrangements in the fourth chromosome of D. pavani.