A protein that binds to the AU-rich sequence in the 3'-untranslated region of sarcotoxin IIA mRNA was purified from a Sarcophaga pupal extract to near homogeneity. The molecular mass of this protein was estimated to be 39 kDa by SDS-polyacrylamide gel electrophoresis. The partial amino acid sequences of two peptides obtained from the 39 kDa protein showed striking similarities to partial amino acid sequences of rat and yeast 3-oxoacyl-CoA thiolase, suggesting that this protein is a Sarcophaga thiolase. In fact, the purified 39 kDa protein was found to have thiolase activity. Moreover, rat mitochondrial 3-oxoacyl-CoA thiolase showed affinity to the AU-rich RNA. These suggest that the RNA binding activity is an intrinsic character of thiolase.