The first episode in the new series of Doctor Who was watched by an average audience of 6.5 million viewers, according to overnight viewing figures.
On a sunny day in parts of the UK, that was down from the eight million recorded by overnight figures on Matt Smith's show debut in April 2010.
The Impossible Astronaut had an average audience share of 36.7%. Figures peaked at seven million.
The episode featured aliens partly inspired by Edvard Munch's The Scream.
BBC One viewers watched as Smith's Doctor was reunited with Karen Gillan's Amy Pond, Arthur Darvill's Rory Williams and Alex Kingston's River Song in 1960s America.
While final consolidated viewing figures - which include playback on recording devices - will push ratings higher, The Impossible Astronaut could turn out to be the least watched series opener since Doctor Who was relaunched in 2005.
The new episode was described by the Daily Telegraph as a "wordy episode which concentrated more on atmosphere than pace and visual thrills".
Thanks, in part to "fizzing dialogue" and "a great concept" for a new monster, it was "a cracking start" to the new series, the paper added.
The Los Angeles Times, reviewing the programme because it is shown on BBC America, said Smith had "screwed into this role good and tight", praising his Doctor as "an ancient child, an unstable mix of authority and impulsiveness".
But Kevin O'Sullivan, writing in the Sunday Mirror, said the episode was called The Impossible Astronaut because it was "impossible to understand" complaining that "this ball of all-round confusion was no way to start a series".
The episode had been written for "strictly sci-fi nerds only", he added.
Saturday's episode began with a dedication to the memory of the late Doctor Who actress Elisabeth Sladen who died of cancer this week at the age of 63.
The actress also appeared in four series of spin-off show The Sarah Jane Adventures on children's channel CBBC, which began in 2007.
At the end of the The Impossible Astronaut, a tribute to Sladen - who starred opposite Jon Pertwee and Tom Baker as the Doctor's assistant between 1973 and 1976 - was screened on CBBC.
Saturday night's tribute featured Smith as well as his Doctor Who predecessor David Tennant.