[citation needed] Between September 1885 and March 1887 he undertook his first voyage of exploration in the Matto Grosso of Brazil starting from Cuiabá, making his first ethnographic collections.

Between 1898 and 1908 he undertook several botanical exploration trips nationwide, primarily in the Eastern region with collaborator Teodoro Rojas.

[citation needed] He researched his collections, publishing numerous studies, some in conjunction with Robert Chodat who accompanied him on a further field trip to Paraguay in 1914.

In 1932, during the Chaco War between Bolivia and Paraguay he established and ran a hospital for the wounded in San Bernardino, working there as a surgeon, and was awarded the honorary rank of Colonel in the Paraguayan Army.

[1] On 28 October 1934 he was awarded Doctorate Honoris Causa by the Universidad Nacional de Asunción in recognition of his outstanding work.

[1] In August 1935, at the end of the Chaco War, the hospital in San Bernardino was closed and Hassler undertook what was to be his final trip to Switzerland.

[5][6] Other ethnographic collections of about 10,000 items made during Hassler's expeditions between 1885 and 1895 were purchased in 1919 by the Museum of Cultures, Basel, Switzerland.

[9] A complete catalogue of these, inclusive of other collectors' related collections from Paraguay, (Catalogus Hasslerianus, 2008-), is being published and made available online.

[12][13][14][15] The collections formed the basis of the description of many new species, published in the work Plantae Hasslerianae in several parts between 1898 and 1907 as well as subsequent studies both by Emil Hassler and by others.