Cal. Code Civ. Proc. § 474
When the plaintiff is ignorant of the name of a defendant, he must state that fact in the complaint, or the affidavit if the action is commenced by affidavit, and such defendant may be designated in any pleading or proceeding by any name, and when his true name is discovered, the pleading or proceeding must be amended accordingly; provided, that no default or default judgment shall be entered against a defendant so designated, unless it appears that the copy of the summons or other process, or, if there be no summons or process, the copy of the first pleading or notice served upon such defendant bore on the face thereof a notice stating in substance: "To the person served: You are hereby served in the within action (or proceedings) as (or on behalf of) the person sued under the fictitious name of (designating it)." The certificate or affidavit of service must state the fictitious name under which such defendant was served and the fact that notice of identity was given by endorsement upon the document served as required by this section. The foregoing requirements for entry of a default or default judgment shall be applicable only as to fictitious names designated pursuant to this section and not in the event the plaintiff has sued the defendant by an erroneous name and shall not be applicable to entry of a default or default judgment based upon service, in the manner otherwise provided by law, of an amended pleading, process or notice designating defendant by his true name.
Ca. Civ. Proc. Code § 474