In this paper, we investigate relationships between different definitions of fuzzy ideals of a fuzzy semigroup. The equivalence of different definitions of fuzzy ideals of a fuzzy semigroup is obtained. We show that the lattice of all phi-fuzzy ideals of a fuzzy semigroup X is a complete sublattice of the lattice of all fuzzy sets in X when phi is an infinitely V-distributive weak pseudo-t-norm on L. A definition of phi-semiprime fuzzy ideal of a fuzzy semigroup is given. A description of all phi-semigroups, in which every phi-fuzzy ideal is phi-semiprime, is obtained. (C) 2009 Elsevier Inc. All rights reserved.