Two multinucleate amoebae species, nowadays belonging to the genus Thecochaos, were never seen alive since 1913. Stained preparations deposited with the British Museum of Natural History (London) by E. Penard remain the only material proof of the existence of these organisms. We recently found an amoeba, identified as Thecochaos fibrillosum, in the soil sample originating from Chernevaya taiga soil in South Siberia (Russia). In total, we have got 15 cells of this species; however, using this amount of cells, we performed light- and electron-microscopic study, made permanent stained preparations, obtained SSU rRNA gene sequence using single-cell DNA isolation technique, and performed NGS sequencing to obtain genomic data for multigene analysis. Its SSU gene is unusually short (1586 bp), has significant losses in many laps, and, to the moment, is the shortest one known among Amoebozoa. The SSU sequence of this species robustly groups within the family Thecamoebidae as a sister to Thecamoeba/Sappinia clade. Multigene analysis either supported this position of Thecochaos or suggested its grouping with Sappinia. This study demonstrates that modern single-cell techniques allow correct and complete study and description of rare and almost non-cultivable amoebae species.