Justin Timberlake enjoyed a ''mellow'' bachelor party.

The singer-turned-actor was joined by a group of friends for a night in Las Vegas followed by a weekend of golf and volleyball in Cabo, Mexico, but the celebrations were low-key as his fiancee Jessica Biel's brother was with him.

A source said: ''It was mellow, intentionally so. He didn't want to get into any trouble. Jessica's brother was with them.''

Justin and Jessica are reportedly planning to wed in Italy in October and Jessica has been busy planning every detail of their big day.

A source told Us Weekly magazine: ''Jessica has claimed she is too busy to plan a wedding but she actually has been very involved in all the details, like every bride.''

Wedding planner Diana Sorenso recently arranged some nuptials at 19th century boutique hotel Palazzo Margherita and claims the star-studded wedding will be held there.

She said: ''I've heard the wedding will be intimate, in the palm-tree lined outdoor garden area, just aside from the stunning black bottom pool and giant pergola.

''There will be flowers galore and it will be extremely elegant - the most stunning the palazzo has seen yet.''

Those expected to attend include Amy Adams, Ellen DeGeneres and Justin's former *NSync bandmates and guests won't have to spend any money as the generous couple are paying for everything.