In interspecific competition between various strains of Drosophila melanogaster and a vermilion mutant strain of D. simulans in continuous populations, heterogeneity among replicate populations and among generations within populations has been observed when the fitness of the melanogaster strain was apparently similar to that of simulans vermilion. Larval and adult numbers vary widely among generations, so that differential effects of population density on fitness components of the two species could be an important factor contributing to the observed heterogeneity. The effects of variation in larval density and in proportions of the two species on viability, sex ratio, developmental period and adult body weight have been determined for the Oregon—R—C wild type strain of melanogaster and the standard vermilion strain of simulans. Thirty treatment combinations of larval density (six levels) and species proportions (five levels–two single species, three mixed) were repeated three times at intervals of 1 week. This variable of time had highly significant effects on all components. Results observed in mixed cultures generally were not predictable from results in single species cultures, particularly at high densities. The general effects of density on both species were similar, but there were significant differences between the species that would cause differences in competitive outcome at different densities. Further, the different components showed different responses to density. Species frequency also had significant effects on each component, but these effects were often different for the two species and were different for each component. As the outcome of competition within a generation was both density and frequency dependent, and given as well the significant effects of time, variation among populations and among generation within populations can be expected. The dynamics of interspecific competition in continuous populations will not be simply predictable.