Soluble esterases of digestive system organs of various developmental stages in the quail (Coturnix coturnix) were resolved by polyacrylamide gel electrophoresis into several molecular forms which were characterized as carboxylesterases, acetylesterases, cholinesterases and esterases sensitive to eserine. 2. The pI of the majority of esterasic activity in several quail and chicken tissues was observed in the range of 5.1-5.6, while the apparent molecular weight in liver extracts was 60,000. 3. The expression of the esterase multiple molecular forms was found to be both tissue- and developmental stage-specific, with electrophoretic patterns becoming more complex in number and/or staining intensity upon hatching and thereafter, especially in liver and intestine.