A new microsensor that can analyze microliter volume samples was used to measure the viscosity of aqueous solutions of antifreeze glycoproteins as a function of temperature and concentration. The results show that at physiological concentrations which naturally occur in the fish, the antifreeze proteins significantly increase aqueous solution viscosity. The probability for ice nucleation is inversely proportional to viscosity. Therefore, the increased viscosity could explain, in part, reports on the beneficial effects of antifreeze glycoproteins during cryopreservation by vitrification. Reducing the probability for ice nucleation could be also beneficial for the survival of cold-water fish in their natural habitat. Millimolar concentrations of antifreeze proteins increase aqueous solution viscosity to values comparable with those of conventional cryoprotectants in molar concentrations.