• by Wayne Friedman , October 1, 2015

Although “The Tonight Show with Jimmy Fallon” won the first week of the TV season in the late-night category, CBS cut into the show’s dominating lead with its new “Late Show with Stephen Colbert.”

NBC’s “Tonight” posted a Nielsen 3.6 million viewers, down 8% from a year ago. But contrast, “Late Show” improved 19% to 3.2 million viewers.

“Late Show” also picked up in key demographics among younger viewers, gaining 60% to a 0.78 rating/4 share among 18-49 viewers; and near 70% among 18-34 viewers, to a 0.5/4.

The “Tonight” show was still ahead among key viewing groups -- a 1.05 rating/5 share in 18-49 viewers -- down slightly from a 1.09/5 in the first week a year ago.

ABC’s “Jimmy Kimmel Live” gave up ground to both shows -- down 13% to 2.5 million total viewers and off 14% to 0.6/3 among 18-49 viewers for the first week.

NBC is also in first place among the later-night talk shows: NBC “Late Night with Seth Meyers” had 1.6 million viewers, up 7%; with CBS “The Late Late Show With James Corden” at 1.4 million viewers, down 7%. “Late Night” also leads in 18-49 viewers -- a 0.5/4, up 17% versus a year ago, while the “Late Late Show” gained 33% to score a 0.4/3.