We investigated the relevance of the relationship between the compactness of ${eta}$-galactosidase inclusion bodies (${eta}$-gal IBs) and their enhanced enzymatic activity with or without the addition of D-fucose (inducer analog) or methyl ${alpha}$-D-glucopyranoside (${alpha}$-MG, catabolite repressor) after induction in the araBAD promoter system of Escherichia coli. Experiments conducted to evaluate the solubilization of ${eta}$-gal IBs in guanidine hydrochloride as well as their trypsin degradation and temperature stability revealed that ${eta}$-gal IBs expressed in response to the addition of D-fucose or ${alpha}$-MG had a looser structure. Additionally, ${eta}$-gal IBs expressed when D-fucose or ${alpha}$-MG was added were more quickly solubilized in guanidine hydrochloride or degraded by trypsin-treatment than those produced when these compounds were not added. Moreover, the activity of ${eta}$-gal IBs expressed when D-fucose or ${alpha}$-MG were added was less stable at various temperatures. Consequently, we deduced that the looser structure of ${eta}$-gal IBs resulted in enhanced enzymatic activity of ${eta}$-gal IBs upon addition of D-fucose or ${alpha}$-MG after induction.