Acid phosphatase is monogenically inherited inDrosophila bipectinata and this gene is monoallelic in the population maintained at Banaras Hindu University, Varanasi. The acid phosphatase band ofbipectinata shows the same mobility in starch gel as does the band determined byAcph-11.05 allele ofmalerkotliana. Hybrid flies betweenbipectinata andAcph-110.5malerkotliana show only a single band with the same mobility as does that of each of the parents. Hybrid flies betweenbipectinata andAcph-11.0 orAcph-10.95 allele ofmalerkotliana show an additional intermediate band in addition to the two parental bands. The additional band is a heterodimer and shows that acid phosphatase is a dimer inbipectinata and thatbipectinata andmalerkotliana acid phosphatase protomers possess identical stereospecific properties. It is concluded thatAcph-1 genes inbipectinata andmalerkotliana are homologous.