Megadeth's lead singer has accused their ex-bassist of using the heavy metal group's name without permission.
Mustaine (l) reformed Megadeth in 2004, without Ellefson (r)
David Mustaine claims he and David Ellefson have an agreement restricting Mr Ellefson's use of the group's name.
Mr Mustaine says an advert Mr Ellefson made for musical equipment contravenes this deal. He is seeking unspecified compensatory and punitive damages.
Founded in 1983, Megadeth disbanded in 2002 but reformed, without Mr Ellefson, two years later.
In the legal action filed on Monday at Los Angeles' Superior Court, Mr Mustaine stated that Mr Ellefson "shall not use the work or mark 'Megadeth' to advertise, market or promote any product or service".
Mr Mustaine claims the ad in Bass Player magazine, which features Mr Ellefson holding a pedal tuner for bass guitars and includes the names of bands he is involved with - including Megadeth - breaks a settlement agreement both parties entered into in February.
Megadeth have since released a new album, The System Has Failed, and a greatest hits compilation, Back to the Start.
Mr Mustaine - who formed the group 22 years ago after he was fired from fellow heavy metal rockers Metallica - disbanded Megadeth in 2002 after an injury to his left hand and arm left him unable to play guitar.
Up until 2002, the group released 13 albums - the most successful of which was 1992's Countdown to Extinction, which reached number two on the US Billboard charts.
They were nominated seven times for Grammy awards.