An employee called police on Monday morning to report the theft after noticing the exhaust components missing from several busses.
Police said the theft is believed to have occurred between 10 a.m. on Saturday and 3 a.m. on Monday.
A total of 21 catalytic converters are missing with a value of approximately $69,600.
There has been a sharp increase in reported thefts of catalytic converters in recent months due to the value of the precious metals inside of them.
Guelph police said more than 60 such thefts have been reported in the city since Christmas.
Police are urging residents to report any suspicious people or vehicles, particularly in remote areas where vehicles might be left overnight.
Vehicle owners should be parking in a garage or in well-lit, highly visible areas, police said.
Anyone with information can call police at 519-824-1212 ext. 7537. Anonymous tips can be left with Crime Stoppers at 1-800-222-8477.