The authors examined fluctuation in microsomal ethoxyresorufin O-deethylase (EROD) activity in the hepatopancreas during the molting cycle of the fiddler crab, Uca pugilator. Results showed that microsomal EROD activity fluctuates significantly during the molting cycle, with the lowest enzymatic activity occurring in the late premolt stage. These results clearly show that molting physiology influences crustacean EROD activity, suggesting that when using crustacean EROD assays in evaluating pollution, only individuals from the same molt stage should be used. The authors propose that the high level of EROD activity in postmolt and intermolt stages is an additional mechanism crustaceans use to prevent any untimely rise in ecdysteroid levels.