Caladomyia n.gen. and the new species C. adalberti, C. castelnaui, C. humboldti, C. kraussi, C. mulleri, C. ortoni, C. poppigi, C. spixi, are described. The genus is divided into two species-groups, the spixi- and the orellanai-groups, containing 8 and 10 species, respectively. The species of the former group is treated in this paper. The orellanai-group will be treated in a separate paper. Morphologically some of the species are very similar. This, together with the minuteness of the species, makes them taxonomically difficult. The pupal exuvium of one species, C. spixi, is known. No females and larvae are known. A key to the males of the genus is presented. Notes on the ecology of the species are given. The known distributional area of the genus is the Amazon area with 18 species known at present. Two species - not discussed here - are known from the Southern part of the Nearctic region. In connection with the study of geographical distribution and phylogenetical relationships of the genus Caladomyia, two paratypes of the African species Tanytarsus bifurcus Freeman, 1958, were studied.