Summary1. Three hundred and fifty‐one specimens of Forest, or, Three‐toed Kingfisher from Malaysia and nearby islands were examined in a quantitative study of the characters distinguishing the dark‐backed taxon,Ceyx erithacus, from the rufous‐backed taxon,C. rufidorsus.2. All the characters were shown to be variable and some form of intergradation between homologous characters was present in the majority of the sample.3. The frequency with which intergradation was found to occur between allo‐patric populations indicated that the two taxa freely interbreed wherever they come into contact.4. It is proposed that the taxa should be regarded as forming one polytypic species,Ceyx erithacus(Linnaeus).5. In a brief review of the palaeogeography of the area it was suggested that the rufous bird evolved in the islands of Malaysia while secondary isolation in northeastern Borneo resulted in the evolution of the dark‐winged rufous‐backed form characteristic of northern Borneo.6. With a fall in sea level at the height of the ice‐age there was a resumption of gene flow between previously isolated populations.7. Currently some of the effect of the exchange of genes is not readily evident due to swamping.8. A brief systematic review of the races ofCeyx erith