variation in amount of enzyme protein in natural populations among strains of drosophila melanogaster each derived from a single fertilized female taken from natural populations there is variation in both alcohol dehydrogenase adh activity and the amount of adh protein the correlation between adh activity and number of molecules over all strains examined is 087 or 096 in late third instar larvae depending on whether the substrate is 2propanol or ethanol with respect to the two common electrophoretic allozymic forms f and s segregating in these populations the ff strains on the whole have higher adh activities and numbers of adh molecules than the ss strains over all strains examined enzyme extracts from ff strains have a mean catalytic efficiency per enzyme molecule higher than that of enzyme extracts from ss strains when ethanol is the substrate and much higher when 2propanol is the substrate one ff strain had an adh activityadh protein ratio characteristic of ss strains