A general rationale for multiple hemoglobin systems of fish has not been found. The family Mugilidae show adult hemoglobin systems which range from one component (Mugil cephalus) to seven (Chelon labrosus and Liza ramada). Basic data on hemoglobins are available for other temperate and tropical species of the family. For two temperate species the ontogenetic variation is also known. Separation of the seven hemoglobins of Liza ramada by means of chromatofocusing has allowed evaluation of the pH dependence of their oxygen affinities. Only slight differences were found. The soundest rationales, in this and in other cases, seem to be related to physiological requirements linked to specific life-styles. Despite discordant reports, the subunit constitution of these complex hemoglobin systems is probably the normal one, i.e. tetrameric, with two α-type and two β-type subunits, although both α and β subunits may differ from one another.