Officers started the contest clean-shaven on Nov.1 and must go without shaving, trimming or shaping their moustaches until Nov. 30, according to the department.
At the end of the month, all contest participants will be photographed, and their pictures will be posted on the police department's Twitter and Facebook pages.
The public is invited to check out the pictures and vote for the best and worst moustaches, organizers said.
Participants donate $10 each and are collecting donations from friends and relatives.
For more information on the contest, visit the Walnut Creek Police Department on Twitter at walnutcreekpd and on Facebook at www.facebook.com/walnutcreekpd.