Following are the top 20 network prime-time shows last week, ranked according to the percentage of the nation's 87.4 million TV households that watched, as measured by the A.C. Nielsen Co. A share represents the percentage of actual sets-in-use tuned to a particular program when it aired.

1 19.9 Cosby Show 40 NBC

2 19.5 Family Ties 37 NBC

3 16.0 Cheers 33 NBC

4 16.2 The Golden Girls 33 NBC

5 15.4 Unsolved Mysteries 30 NBC

15.4 Beverly Hills Madam 27 NBC

7 15.3 Molly Dodd 28 NBC

8 15.2 My Mother's Secret Life 28 ABC

9 14.5 Valerie 28 NBC

14.5 Growing Pains 27 ABC

11 14.3 Amen 29 NBC

14.3 L.A. Law 26 NBC

13 14.1 Miami Vice 29 NBC

14 14.0 Hunter 28 NBC

15 13.8 60 Minutes 29 CBS

16 13.7 Designing Women 23 CBS

17 13.6 ALF 28 NBC

18 13.4 Newhart 24 CBS

19 13.0 20/20 24 ABC

20 12.9 Matlock 25 NBC

NBC won in prime time last week with a 12.8 Nielsen rating and a 25 percent share of the national audience for its 28th consecutive win, the longest winning streak in NBC's history. ABC was second at 9.6/19, hugged by CBS in third at 9.5/18.

The top seven of the top 10 shows were all from NBC's stable, with "The Cosby Show" and "Family Ties" leading the pack.

Among network news programs, "NBC Nightly News" remained ahead with a 9.8/22 for its 17th straight week, as ABC trailed with a 9.1/20 and CBS followed closely behind with a 9.0/20.

NBC's "The Days and Nights of Molly Dodd" scored its highest ranking ever this week at seventh and has already been renewed for the fall.

The bottom five, beginning with last and 67th place, were an episode of "The Ellen Burstyn Show," followed by "The Charmings" at 66th, "Dallas," "Our World," and finally "Sledge Hammer!" at 63rd.