On January 30th, 1976 Dr. Maas Geesteranus, Senior Mycologist, left the Mycological Department of the Rijksherbarium, Leiden, at the mandatory retirement age of sixty-five. This provides us with an occasion for outlining his career and commemorating his scientific achievements. Rudolf Arnold Maas Geesteranus was born on January 20th, 1911, at the Hague. Before he was a year old the family went to the erstwhile Dutch East Indies and he remained there until 1929. As a growing boy he was deeply impressed by the richness and variety of nature in the tropics.