Wilhelm Michaelsen, also sometimes identified as Johann Wilhelm Michaelsen or [Johann] Wilhelm Michaelson, was a German zoologist. After completing his schooling he first studied mechanical engineering, but later became a post-graduate student in the natural sciences. In 1887 he was appointed as research assistant in the Hamburg Zoological Museum, where he was later promoted to chief curator. He worked there until the time of his death.
Michaelsen specialised in the study of the Order Oligochaeta (which includes earth- and mud-worms), though he also published on the Order Polychaeta (lob-worms, or lug-worms) and the Order Ascidiacea (including sea-squirts). He visited the southern regions of Africa, Australia and South America in order to study the distribution of particularly the Oligochaeta and found evidence in support of Alfred Wegener's theory of continental drift. Some of his major publications are: Oligochaeta (1900, 575 pp.); Die fauna suedwest-Australiens (5 vols, 1907-1930), based on the Hamburg Research Expedition to South-West Australia in 1905; Oligochaeten von tropische und suedlich-subtropischen Afrika (2 vols, 1912-1913); and Beitraege zur Kenntnis der Meeresfauna Westafrikas (Hamburg, 1914-1932, 3 vols).
Most of Michaelsen's more than 200 publications appeared under the name W. Michaelsen. However, in a few others he is identified as either W. Michaelson, J.W. Michaelsen, or J.W. Michaelson. A few of his publications dealt with material from southern Africa, for example 'The Oligochaeta of Natal and Zululand' (Annals of the Natal Museum, 1913). During a research visit to German South West Africa (now Namibia) in 1911 he collected reptiles that were described by Franz Werner in a two volume work by Michaelsen, Beitraege zur Kenntnis der Land- und Suesswasserfauna Deutsch-Suedwestafrikas: Ergebnisse der Hamburger deutsch-suedwestafrikanischen Studienreise 1911 (Hamburg, 1914-1916 and 1922-1928, 2 vols, issued in parts). Years later he wrote 'Opistophore oligochaeten aus dem mittleren und dem suedlichen Afrika hauptsaechlich gesammelt von Dr F. Haas waehrend der Schomburgk-Expedition 1931-1932' (Abhandlungen Senckenbergischen Naturforschenden Gesellschaft, 1933). The next year he described 'The Ascidians of the Cape Province of South Africa' (Transactions of the Royal Society of South Africa, 1934).