Juliomys ossitenuis was previously known from the Atlantic Forest of southeastern Brazil, where it occurs in Dense Ombrophilous Forest. The record of the species in Floresta Nacional de Piraí do Sul, in Paraná state, confirmed by morphological and cytogenetic analyses, extends its distribution about 200 km southwards. This is the first record of the species in the state and in an area of Mixed Ombrophilous Forest (Araucaria Forest).
