McGill's has introduced a 10% rise on all their weekly and monthly tickets including the company's Go!Zone range for adults, students and children.
The latest rises were introduced yesterday and triggered an angry backlash from several passengers.
One 35-year-old office executive who travels daily from Renfrew to Glasgow city centre said: "This is the third rise since last January which has seen the cost of my ticket increase from £45.50 to £55.
"The company is blaming a rise in operational costs but they haven't been spending the extra income on wages. Bus drivers tell me they haven't had a wage rise in two years.
It's beginning to look as if they are taking advantage of their monopoly which just isn't right."
Greenock-based McGill's took over its main rival Arriva Scotland West almost two years ago.
A spokeswoman said: "McGill's has endeavoured to keep price increases as low as possible. To illustrate, McGill's takeover of Ar riva happened in March, 2012, and it was only in September 2013 - more than 18 months later - that certain fares actually increased above those charged by Arriva."
She admitted this week's increase was the third rise in a year but insisted: "The reality is that McGill's maintained lower fares for its passengers.
"The suggestion that we are abusing our position when we are the only bus operator in some areas is simply untrue.
"McGill's passengers still have access to some of the cheapest bus fares in Scotland and our multi-journey tickets continue to offer excellent value for money.
"For less than £2 per day, passengers can make unlimited journeys across our entire Renfrewshire network when they buy a monthly Go!Zone1 ticket.
"A similar monthly Go!Zone5 ticket allows for unlimited travel between Gourock and Glasgow city centre for less than £3.40 per day.
"Students and children travel for even less."
Bus bosses blame the increases on rising labour and material costs and a cutback in government subsidies towards the cost of concessionary fares
But despite the need to generate extra income they say the firm will go-head with plans to carry out £3million of improvements to their fleet.