We have previously shown that homozygous mutants of Drosophila melanogaster deficient in the oxygen radical scavengers, CuZn superoxide dismutase or urate, are adult viable and yet hypersensitive to the oxygen radical-generating agent, paraquat. Thus, paraquat could be used as a selective agent to identify adult-viable mutants potentially defective in other, perhaps unknown, oxygen defense functions. Here we report the successful use of paraquat hypersensitivity in the isolation of X-linked, ethylmethanesulfonate-induced mutations affecting oxygen defense in Drosophila melanogaster. Two paraquat hypersensitive mutants were identified that, by complementation analysis, were shown to be new alleles of the maroon-like gene. In addition to paraquat hypersensitivity, both alleles confer a maternally affected dark brown eye color and a complete lack of enzymatically active xanthine dehydrogenase, both of which are characteristic phenotypes of known maroon-like alleles. We conclude that the lack of xanthine dehydrogenase in these mutants leads to the absence of urate, which is the proximate cause of paraquat sensitivity. Because our search for such mutants on the X chromosome revealed two alleles of only a single selectable gene, we anticipate that the total number of major oxygen defense genes in the complete Drosophila genome may not be large.