In 1967, he became curator at the Rijksmuseum van Natuurlijke Historie[2] where he started the dragonfly collection.

[5] Next, he went to the University of Basel, and in 1935 obtained his doctorate magna cum laude[1][2] for a thesis on the fauna and ecology of the Swiss Jura.

[7][8] In 1938, Geijskes started to work as an entomologist for the Landbouwproefstation (Experimental agricultural station) in Suriname.

[2] In 1958, he led an expedition to the Tafelberg with Rudi Kappel to examine the savannah around the mountain, and investigate whether an airstrip could be built there.

[14] The successful construction of an airstrip led to Operation Grasshopper in 1959 which mapped the natural resources in the interior.

[15] On 2 May 1965, Geijskes returned for the Netherlands,[16] and in 1967 started to work as curator at the Rijksmuseum van Natuurlijke Historie (nowadays Naturalis).