It was Harold Fleming who first drew the attention of linguists to the Bayso language. Reports concerning the existence of such a people on Gidicho Island in Lake Abaya go back to the beginning of the century and descriptions of various cultural and economic aspects of the life of the Bayso and other inhabitants of Gidicho appeared subsequently; but concerning the language nothing was available until the publication of Fleming's germinal article in 1964. The latter followed a brief period of work on the island with a Bayso speaker. The 216-item vocabulary list and brief indications of the grammar have nevertheless proved tantalizingly suggestive to everyone working on questions of genetic relationships within Cushitic. Alongside the Bayso material the article also presented the first data on the Rendille language of north-east Kenya. Fleming's conclusions that Bayso, Rendille, Boni, and Somali constitute a Macro-Somali group were seen as linguistic support for H. S. Lewis's hypothesis of a south Ethiopian (rather than a northern Horn of Africa) origin for the Somali. Since the initial presentation of these proposals, linguistic research into the Dasenech (Galab), Elmolo, and to a limited extent Arbore, languages has led German linguists to propose an extension of Fleming's Macro-Somali to include these languages. Until very recently the limited nature of the data available on many of the relevant languages had tended to detract from the strength of arguments about their internal relationships. Heine's reports of his recent researches on Rendille, Elmolo, and Boni, as well as his work on the diachrony of the ‘Sam languages’ have done much to advance our knowledge. To date, however, the material available on Bayso has remained restricted to what was presented by Fleming in 1964. The primary purpose of the present paper is to set out the results of research conducted for a period of 17 days during September 1976.