SummaryA brief account of some of the recent advances in our knowledge of the general biology, taxonomy and economic importance of the genusMerluccius, which have resulted mainly from the work of Hickling and of theDiscoveryinvestigations, has been given by way of introduction to a more detailed discussion of its distribution, and in the hope of demonstrating the great interest of hake from a general biological point of view.The seven species of true hakes (i.e. notUrophycisspp.) distinguished by Norman (1937), and their normal distributional limits, are:Merluccius merluccius(Linnaeus)From the Norwegian Rinne southwards along the edge of the continental shelf west of Europe to Dakar. Mediterranean.M. hubbsiMariniFrom Magellan Straits northwards overthe Patagonian continental shelf to southern Brazil.M. productus(Ayres) Southern California to north‐western Alaska. ? China.M. gayi(Guichenot) Southern Chile to Paita in Peru, and possibly farther north.M. bilinearis(Mitchill) South coast of Newfoundland southwards to North Carolina and, rarely (in deep water only) to Florida and the Bahamas.M. capensisCastelnauOff South Africa, from Angola to Natal.M. australis(Hutton)Chatham Island, South Island of New Zealand and northwards to East Cape on North Island. AChallengerspecimen from the Magellan region.There is a remote possibility that certain aberrant specimens secured in deep tropical waters off West Africa and in Panama Bay may represent two further distinct species, bridging the gaps betweenM. merlucciusandM. capensisand betweenM. gayiandM. pro‐ductus.It is more probable that they represent odd stragglers (at most only racially distinct) ofM. merlucciusandM. gayirespectively.It has been shown that all the best‐known species ofMerlucciusconform in a striking manner to the same distributional pattern in relation to the major hydrological features, within the limits of their normal range. Where relatively cold currents flow towards the equator in the warmer half of the normal habitat of any one of these species, the range of the species is extended in that direction; but if a relatively warm current is flowing polewards, the range in the direction of the equator is restricted. In the colder half of the normal habitat of each species the converse relationship holds good.Surface isotherms have been used as the most reliable general criterion symptomatic of the environmental complex that leads to this type of distribution because in many parts of the world more detailed hydrological data are not yet available; but it is emphasized that other factors, more or less intimately interrelated with the direct effect of temperature, are also involved.The distribution of the genusMerlucciusseems to offer a good example of the wider aspects of the phenomenon of ‘organic polarity’ discussed by Wimpenny (1941), while the bionomics of the better known individual species show some more detailed aspects of it with great clarity.I am greatly indebted to Dr N. A. Mackintosh, Director of Research,DiscoveryInvestigations, for permission to publish this review while I am still engaged on other work for theDiscoveryCommittee, and to Miss E. Humphrey, who re‐drew the chart for publication; also to the Director and staff of the Marine Biological Association's Plymouth Laboratory (where the article was written), especially the Librarian, Miss Sexton. Talks with Messrs Hickling, Carruthers, Deacon and Wimpenny have been most helpful and