Only last month, Alexander Dennis won a £50m order from bus and rail company FirstGroup's Perth-based rival, Stagecoach.
The latest order boosts further the job security of Alexander Dennis's workforce of about 900 at its Falkirk headquarters.
Bill Simpson, group corporate affairs director of Alexander Dennis, said the firm now had a record order book of nearly £300m.
Alexander Dennis has enjoyed rapid growth under chief executive Colin Robertson, and is pursuing international expansion.
Mr Simpson said of the latest order: "It consolidates a fantastic working relationship we have got with FirstGroup, (and) underpins manufacturing at our Falkirk facility, along with other substantial orders we have won."
In November, Alexander Dennis unveiled orders for more than 600 buses from Hong Kong.
Mr Robertson said: "We are delighted to be supplying almost 50% of FirstGroup's total new vehicle requirement this year."
He said he was particularly pleased that 142 of the vehicles to be supplied by Alexander Dennis to FirstGroup would be the "ultra-fuel-efficient" Enviro300 single-deck vehicles.
Mr Simpson put Alexander Dennis's share of the UK bus market at nearly 50% and highlighted its expansion in China and the Hong Kong special administrative region, Malaysia, Australia, New Zealand, and North America.
Referring to the FirstGroup order, he added: "If you don't get the support of your local businesses, you don't really go anywhere."
FirstGroup yesterday announced orders for a total of 464 vehicles, worth £76m, for delivery in its 2013/14 financial year. It said all of the vehicles would be fitted with wi-fi, so customers could "stay connected".
The second-largest of the latest orders from FirstGroup has gone to Wrightbus of Northern Ireland.
Alexander Dennis was rescued from administration in 2004 by a consortium including Stagecoach co-founders Sir Brian Souter and Ann Gloag, Sir Angus Grossart's Edinburgh-based Noble Grossart merchant bank, and metals-to-property entrepreneur Sir David Murray.