The CDU now has 530,755 members to the SPDs 529,994, general secretary Ronald Pofalla told reporters in Berlin, putting the party in pole position for a general election expected in 2009.
The two parties currently govern in an uneasy “Grand Coalition” in Berlin, with the junior SPD party trailing badly in the polls under leader Kurt Beck.
However, both parties have lost members over the past two decades, the figures also reveal.
The CDU reached its zenith in 1983, when it had 735,000 registered members.
The SPD reached a record of 1.02 million in 1976.
The SPD ruled for two terms in coalition with the Green party until 2005's inconclusive election saw the CDU and SPD forced to form a government.
Germany's traditional four-party structure of the free-market Liberal Party, the centre-right CDU, the centre-left SPD and the environmentalist Greens has been thrown off balance by the rise of the hard-left Left Party, formed in 2007 out of the remnants of the old East German communist party and leftwing defectors from the SPD.
The CDU lost 6,000 members in the first half of 2008, a loss Pofalla put down to the deaths of older members.
Unless the “Grand Coalition” collapses before then, a general election is expected in September 2009.