The drivers went on strike against Veolia Transportation on Saturday over wages and benefit disputes. The two sides had been negotiating for nearly two years but could not reach a deal.
The Phoenix and Tempe drivers are represented by the Amalgamated Transit Union under separate contracts.
They drive 50 of the 101 routes served by the regional Valley Metro transit system. Veolia says service will be badly affected.
Riders are urged to make other arrangements or check their route on the Valley Metro website. Valley Metro serves more than 200,000 bus riders a day.
Metro light rail is not affected.
