insert into / select from / serial problem

I am using a insert statement like:

INSERT INTO newtable
SELECT field1, field2 FROM anothertable

newtable structure is: serial, varchar, varchar

What syntax do I use to insert the serial field? Ive tried something
like:
INSERT INTO newtable
SELECT nextval('newtable_id_seq'), field1, field2 FROM anothertable
but that doesnt work

Any ideas? Greatly appreciated!