'The Beast Below' failed to top last week's series premiere, but was the most-watched programme during primetime.
Also on BBC One last night, Over The Rainbow put in 5.35m (25.1%) at 7pm, 6.03m (26%) watched The National Lottery: Who Dares Wins at 8.15pm, and the 9pm screening of Casualty was seen by 5.61m (24%).
Dad's Army interested 1.81m (8%) for BBC Two at 8pm, then the live coverage of Golf: The Masters averaged 2.01m (10.6%) between 8.30pm and midnight.
Harry Potter And The Order Of The Phoenix claimed 1.21m (5.3%) between 7pm and 9.30pm, then Michael Winner's appearance on Piers Morgan's Life Stories drew 1.98m (9.3%).
Channel 4's Pearl Harbour: Mystery of the Killer Subs had 1.21m (5.3%) at 8pm, then the audience climbed to 1.39m (9.2%) during The 100 Greatest Stand Ups 2010 between 9pm and 1.15am.
CSI and spinoff New York attracted 694k (2.9%) and 1.73m (7.9%) for Five at 8.30pm and 9.30pm respectively.
Overall, BBC One dominated primetime with an average share of 23.7% to ITV1's 19.6%. BBC Two was third with 7.9%, followed by Channel 4 with 6.8% (+1: 0.6%) and Five's 4.8%.
BARB ratings data supplied by Attentional