1. The relationship between Km and assay temperature was examined in three tropical and two temperate Drosophila species, and in the cosmopolitan species, D. melanogaster, for isocitrate dehydrogenase and acetylcholinesterase. 2. For both enzymes Km patterns were similar among species from the same habitat, and different between habitats. No such parallelism was seen with respect to thermal inactivation. 3. The Q10 values in general reflected temperature dependent changes in Km, but exceptions were noted.