As the highway crosses into Kalkaska County it crosses the Manistee River. M-66 runs through rolling hills in woodlands through the unincorporated farming community of Lodi north to an intersection with M-72. The two highways travel west together over the Great Lakes Central Railroad before turning north and merging with US 131 on a route parallel to the rail line. US 131/M-66/M-72 follows and crosses a branch of the Boardman River along Cedar Street through downtown Kalkaska. North of the central business district, M-72 separates to the west and US 131/M-66 crosses through the Pere Marquette State Forest on the way to Antrim and Mancelona in Antrim County. The highway follows Williams Street through the twin towns, meeting the southern terminus of M-88 and western terminus of C-38 at the intersection with State Street in Mancelona. M-66 separates from US 131 and follows Mancelona–East Jordan Road out of town.