The material studied comes from recent captures in the State of Rondônia. The material will be deposited in the Invertebrate Collection of the National Institute for Amazonian Research (INPA). All specimens (five females) were firstly preserved in alcohol 100% , the thorax and abdomen are somewhat crumpled and the pilosity is partly missing. Traditionally, external characters are sufficient to determine species in Tabanus (Fairchild 1983, Fairchild 1984, Carmo and Henriques 2019) and the use of terminalia has not been very productive below the genus level in Tabanidae (Krolow and Henriques 2009, Krolow and Henriques 2010). For these reasons, the specimens were not dissected. The morphological terminology follows Cumming and Wood (2017).

Specimens were examined and digitally photographed through a stereomicroscope LEICA M205C coupled with a LEICA DFC 295 camera and the images were processed using the software Leica Application Suite LAS V3.6. Frons indices: Frontal index = frons height/ frons width at base; Divergence index = frons width at vertex/ frons width at base.

The new species described here possibly is related to T. nebulosus species-group, which has nine species and one subspecies, all Neotropical with occurrence records for Brazil or neighbouring countries, with the exception of T. punctipleura Hine, 1920, restricted to Costa Rica and Panama. According to Fairchild (1984) T. nebulosus species-group “has species with a moderately narrow and generally nearly parallel sided frons with slender clavate or ridge-like callus and fairly prominent black pilose prescutellar spot”. T. rondoniensis sp. n. can be distinguished from the other species of the T. nebulosus group by the diagnostic characters (unpatterned eyes, antennal scape inflated, wider than first flagellomere, a black pilose prescutellar spot, a median row of large pale pilose and pruinose connected triangles on tergites 2–5) and, additionally, by the wing without contrast marks; abdomen without dorso-lateral pale patches or dark median integumentary markings; palpus not reduced; unpainted costal cell and brown legs. Tabanus comosus Stone, 1944, which has an enlarged scape and divergent frons above, can be distinguished by the presence of dorsolateral pale patches on the tergites, absent in T. rondoniensis. T. rondoniensis can be confused with T. rubripes Macquart, 1838, which also has a black prescutellum and light median triangles on the abdomen, but it has two-banded eyes, frons narrower (frontal index ca. 9), a long appendix on the fork of vein R₄₊₅, cell r₅ narrowed at apex or closed and the median pale triangles in the abdomen are not connected. Currently, 26 species of Tabanus have been registered for the State of Rondônia, of which three belong to the T. nebulosus species-group (Henriques and Gorayeb 1993, Henriques 1997,Coscarón and Papavero 2009, Carmo and Henriques 2019). T. rondoniensis is the 27th species of Tabanus and the fourth of the T. nebulosus species-group for the State of Rondônia.