Genetic heterogeneity in populations of D. melanogaster has been described for resistance to long-term exposure to CO2 (4 to 5 hours). Crosses between inbred strains, and between strains set up from single inseminated females collected in the wild show the importance of additive genes. Genetic activity for resistance and sensitivity was found on the X, 2 and 3 chromosomes.The mechanism of resistance was shown to be an anoxia effect since the effect of an N2 atmosphere was the same as that of CO2. A study of 18 strains collected in the wild revealed a positive correlation between metabolic rate as measured by O2 uptake and mortality under CO2, and negative correlations were found between body weight, and both mortality under CO2 and metabolic rate. These results are consistent with an anoxia effect. A further variable correlated with body weight is resistance to desiccation. Thus the anoxia effect is correlated with factors determining the distribution of the species in the wild.