We inquire further into the properties on fuzzy closed ideals. We give a characterization of a fuzzy closed ideal using its level set, and establish some conditions for a fuzzy set to be a fuzzy closed ideal. We describe the fuzzy closed ideal generated by a fuzzy set, and give a characterization of a finite-valued fuzzy closed ideal. Using a t -norm T , we introduce the notion of (imaginable) T -fuzzy subalgebras and (imaginable) T -fuzzy closed ideals, and obtain some related results. We give relations between an imaginable T -fuzzy subalgebra and an imaginable T -fuzzy closed ideal. We discuss the direct product and T -product of T -fuzzy subalgebras. We show that the family of T -fuzzy closed ideals is a completely distributive lattice.