This species’ status is subject to substantial conjecture. Discovered as recently as 1982, when a male was trapped and collected at an isolated woodlot near sea level in central Rio de Janeiro, the species went unrecorded again until 1994. Since then birds apparently matching the holotype of Myrmotherula fluminensis have been observed at the nearby REGUA reserve, in the same state. However, these birds are apparently vocally indistinguishable from White-flanked Antwrens (Myrmotherula axillaris luctuosa), and it seems possible, if not probable, that the Rio de Janeiro Antwren represents merely a variant plumage of the commoner species, as first suggested in 1997, or that the name fluminensis perhaps pertains to a hybrid between White-flanked Antwren and the globally threatened Unicolored Antwren (Myrmotherula unicolor). It is planned to test these hypotheses genetically, using the holotype. At present BirdLife International treats the Rio de Janeiro Antwren as a Critically Endangered species given that its known range is minuscule and is confined to the extensively deforested lowlands, although the habitat at both localities from which the species has been recorded is second growth. Rio de Janeiro seems to follow mixed-species foraging flocks, remaining low above the ground, searching for insects on branches and leaves. It has not been found above 200 m elevation.