ABSTRACT The diversity of mRNA half-lives was investigated in a differentiated insect tissue, the silkmoth galea, which is specialized for production of a characteristic protein, the zymogen of the proteolytic enzyme, cocoonase. It appears that the differentiation-specific cocoonase mRNA is uniquely stable (half-life ca. 100 h). In addition, the non-specific mRNAs (average half-life ca. 2.5 h) show considerable variability in terms of stability. General methods are described for evaluating half-lives of mRNAs coding for distinguishable semipurified proteins. Some of the limitations of these procedures are also discussed. The observations that mRNA half-lives are variable, and that the differentiation-specific mRNA is unusually stable, have numerous developmental implications. A key role is attributed to differential mRNA stability in a quantitative model of developmental regulation.