Greenock-based McGill's, owned by the Easdale family for a decade now, has agreed to buy transport company Arriva's West Scotland division in a deal which will double its size.
The £10million deal will add 165 buses to McGill's fleet and about 380 employees.
It means the firm will become the third-biggest bus operator in Scotland.
McGill's, which currently operates most of its services in Inverclyde and has a base at Barrhead, Renfrewshire, will take on depots at Johnstone and Inchinnan together with the associated routes and buses. Following completion of the deal, expected to go through by March, McGill's will have more than 700 staff and in excess of 300 buses.
Managing director of McGill's, Ralph Roberts, pledged: "The senior team at McGill's will work closely with Arriva Scotland West management to ensure as seamless a transfer for staff and ultimately, for passengers, as possible."
Mr Roberts was managing director of Arriva Scotland West until he joined McGill's in March last year.
Nigel Featham, Arriva's regional managing director, said: "The team at Arriva Scotland West are a real part of their local community and have strived over the past 18 months to put the business on a stronger commercial footing in the face of increasing costs and the wider economic impact of the recession.
"Both their professionalism and custo- mer focus has been commendable.
"There was, however, limited opportunity to grow this relatively isolated part of Arriva's UK bus business and the offer received represents good value for Arriva."
McGill's have been expanding in Arriva's Renfrewshire territory for several years, acquiring smaller operators and starting high-profile routes such as the X23 from Erskine to Glasgow and the 38 from Paisley to Glasgow.