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The structural properties and the conformational dynamics of antarctic fish Notothenia coriiceps and mouse metallothioneins were studied by Fourier-transform infrared and fluorescence spectroscopy. Infrared data revealed that the secondary structure of the two metallothioneins is similar to that of other metallothioneins, most of which lack periodical secondary structure elements such as α-helices and β-sheets. However, the infrared spectra of the N. coriiceps metallothionein indicated the presence of a band, which for its typical position in the spectrum and for its sensitivity to temperature was assigned to α-helices whose content resulted in 5% of the total secondary structure of the protein. The short α-helix found in N. coriiceps metallothionein showed an onset of denaturation at 30°C and a Tm at 48°C. The data suggest that in N. coriiceps metallothionein a particular cysteine is involved in the α-helix and in the metal-thiolate complex. Moreover, infrared spectra revealed that both proteins investigated possess a structure largely accessible to the solvent. The time-resolved fluorescence data show that N. coriiceps metallothionein possesses a more flexible structure than mouse metallothionein. The spectroscopic data are discussed in terms of the biological function of the metallothioneins. Proteins 2002;46:259–267. © 2002 Wiley-Liss, Inc.