It is known that iron is deposited in the enameloid of some teleost fishes, although its biological significance has not been clarified. In the present investigation, a quantitative analysis of iron in the enameloid of fishes of a primitive suborder, the Balistoidei, and an advanced suborder, the Tetra-odontoidei, of the Tetra-odontiformes of marine teleosts was performed by means of the electron microprobe. The results indicated that the enameloid of Balistoidei contained from 0.4-13.5% iron at its surface layer, whereas that of Tetra-odontoidei was very low in iron, which could not be discriminated from the background value of the emission intensity. The enameloid of three perciform species belonging to the Acanthuridae-from which the Tetra-odontiformes are considered to have been derived-also contained high iron (2. 7 - 3.9%) throughout its entire layer. The iron concentration in the enameloid seemed to be related to the phylogeny of fishes rather than to their environmental water and feeding habits, and it is believed that the mechanisms of iron concentration into the developing enameloid have been lost during evolution from the Achanthuridae to the Tetra-odontoidei. Since a similar phenomenon has been previously observed with respect to the fluoride concentration in the enameloid of the same fishes (Suga et al., 1981a), it is speculated that the concentrations of iron and fluoride, which have originally no chemical correlation, have some special biological significance, although the timing and distribution pattern of their deposition are completely different.