Reginald S. Guest was a son of Herbert Guest and his wife Mary and a younger brother of Herbert Melville Guest*. He was a foundation member of the Grahamstown Natural History Society (1884-1887?) and at its first meeting in December 1884 explained to members how the praying mantis catches its prey. At a subsequent meeting in November 1885 he exhibited the skulls of a baboon and a dassie, and in February 1886 the larvae of the "so-called Kaffrarian silk worm". The larvae occurred in the Grahamstown region, feeding on fig trees. They resembled the cultivated silk worm, Bombyx mori, but had a much smaller coccoon from which very good silk could none the less be wound. [Presumably one of the few true silkmoths that occur in South Africa and feed on wild fig trees, Ocinara sp. (family Bombycidae).] The next year Guest was secretary of the society.
In 1886 he donated a perfect nest of a woodpecker to the Albany Museum, the specimen being new to its collection. Years later, in 1899, he sent the museum a bird from Naauwpoort [Noupoort?]. Meanwhile he had written Poultry farming, a pamphlet (No. 9 of 1898) published by the Cape Department of Agriculture. In later years he turned to genealogical research and published a two volume History of the Guest family (Grahamstown, 1915 & 1938). He was married to Lucy Edith Francis, with whom he had at least one son.