This paper constitutes the first account of the tense morphology of Mewahang, a hitherto undescribed Trans-Himalayan language (Upper Arun, Kiranti). The paper discusses non-past marking in the western dialect of Mewahang, presenting and illustrating its different allomorphs and relating them historically by internal reconstruction. Furthermore, comparative notes on non-past marking in the other Upper Arun languages, Eastern Mewahang, Yamphu and Lohorung, are added. It is shown that a non-past morpheme * -yuk can tentatively be reconstructed for Proto-Upper Arun, but that at the same time the Upper Arun languages show a strong tendency to quickly grammaticalise new non-past morphology, and that the reconstructed morpheme is only retained in traces in Eastern Mewahang and Yamphu. The reconstruction of *- yuk for Proto-Upper Arun is the first empirical substantiation of the Upper Arun branch of Kiranti, since it probably constitutes a diagnostic innovation of this group of languages.