Strike: The Cuckoo’s Calling is the first of three TV adaptions of the Cormoran Strike series of books, written by J.K. Rowling under the pseudonym Robert Galbraith.
The first episode last night averaged 5.5 million viewers in the 9PM hour, beating the return of ITV's Victoria in the same slot which had 4 million.
Victoria returned down 1.4 million viewers on the series one opener in 2016, and 800,000 below the Series 1 average, based on overnight figures.
Strike returns tonight (Monday) on BBC One whereas Victoria continues next Sunday night.
The most watched show on TV last night was BBC One's Diana doc, 7 Days, which averaged 5.6 million viewers. The one-off special comes a month after an ITV show also looking at Diana's death, Diana, Our Mother: Her Life and Legacy, averaged 7.4 million.
Earlier Countryfile averaged 4.5 million viewers on BBC One while on ITV The Best Adverts In The World had just 1.6 million viewers in prime time.
Elsewhere, Dragon's Den continued with 1.8 million on BBC Two while The Mel and Sue Story had 600,000 viewers on Channel 5.