Book excerpt: An aerial survey of the northern Richardson Mountains' Dall's sheep (Ovis dalli dalli) population was conducted between 19 and 22 June, 2017. The survey was conducted by helicopter on high mountain ridges keying in on post lambing habitat. The survey area blocks were established in 1984 and spans the border between the Northwest Territories and Yukon Territory. A total of 647 sheep were observed, of which 506 were adult sheep and 141 were lambs. Of the adult sheep, 323 were nursery sheep and 183 were rams. This is an increase in the size of the population compared to the last survey in 2014 (496 total sheep). The average lamb to nursery sheep and ram to nursery sheep ratios for this population were both healthy at 44:100 and 57:100, respectively. It is worth noting however that these ratios varied among survey blocks.