Ivan Klasnic began where he left off last season by firing Bolton into the third round of the Carling Cup with a 1-0 win.
The Croatian was handed his first start since rejoining Wanderers in the summer as one of nine changes made by boss Owen Coyle and the striker sank Southampton with an opportunist first-half strike.
Coyle was also indebted to Hungarian youngster Adam Bogdan, who was given his debut in goal and produced a string of fine saves as Bolton, with an eye on Premier League survival, avoided a shock.
Saints started well though they found themselves behind in the 31st minute when former Southampton striker Kevin Davies knocked down Sam Ricketts' cross for Klasnic to prod home.
The hosts momentarily thought they had a chance to equalise when Adam Lallana tumbled in the area but referee Keith Hill booked the young midfielder for a dive.
Saints continued to plug away after the break and continued to find gaps in Bolton's defence with Jason Puncheon firing narrowly wide, while in stoppage time, Bogdan denied Saints by finger-tipping sub David Connolly's effort over the bar.