The Phox S strain of Drosophila melanogaster is an electrophoretically slow variant found in a wild population at Victoria, Australia. Prophenol oxidase isoform A1 from PHOX-S was purified and characterized biochemically and genetically. The purified fraction of A1 from PHOX-S showed a homodimer with a molecular weight of the subunit of approximately 77 kDa. The Phox S strain was temperature sensitive in vivo in culture, and the purified protein was thermolabile in vitro. By the deletion mapping method, the Phox S locus was cytologically estimated to be at the location 55-A on the right arm of the second chromosome and 79.6 genetically. These data show that PHOX-S is an electrophoretic variant of MOX and that PHOX-S is the first thermolabile protein found in invertebrate prophenol oxidase.