Liam Trotter helped Bolton scrape nervously past Grimsby 1-0 and chalk up a seventh successive win in all competitions.
The League One high-flyers looked set for a comfortable day at the Macron Stadium when former Mariners loanee Trotter headed home Zach Clough's corner for his second goal of the season after 20 minutes.
Several chances went begging for Phil Parkinson's side and Jamie Proctor rattled the post but their failure to put the game beyond the plucky League Two outfit nearly cost Bolton dear.
Grimsby came into their own after the break. Brandon Comley was denied by a brilliant Mark Howard save at point-blank range and Scott Vernon headed narrowly wide from Danny Andrew's left-wing cross.
James McKeown made two important saves from Sammy Ameobi and Proctor as Bolton - playing their first game at this stage of the FA Cup since 1992 - made a rare foray into opposition territory.
Backed by nearly 2,000 travelling fans, Dave Moore's side pushed hard for a replay and nearly snatched one after substitute Rhys Browne stabbed a late effort towards goal but it was tipped round the post by Howard.