Bulgaria has committed to implementing the Schengen Agreement, although it has not yet done so. For citizens of the European Union (EU) or the European Free Trade Association (EFTA) (i.e. Iceland, Liechtenstein, Norway and Switzerland), an officially recognised identity card (or passport) is sufficient for entry. For other nationalities, a passport is usually required for entry.
Travel to or from another country (Schengen or non-Schengen) from or to Bulgaria is (as of now) subject to the usual immigration controls, but customs controls are abolished for travel to or from another EU country.
Check with your travel agent or the local Bulgarian embassy or consulate.