The yeast Candida albicans is the most important fungal pathogen of humans and a model organism for studying fungal virulence. Sequencing of the C. albicans genome will soon be completed, allowing systematic approaches to analyse gene function. However, techniques to define and characterize essential genes in this permanently diploid yeast are limited. We have developed an efficient method to create conditional lethal C. albicans null mutants by inducible, FLP-mediated gene deletion. Both wild-type alleles of the CDC42 or the BEM1 gene were deleted in strains that carried an additional copy of the respective gene that could be excised from the genome by the site-specific recombinase FLP. Expression of a C. albicans-adapted FLP gene under the control of an inducible promoter generated cell populations consisting of > or = 99.9% null mutants. Upon plating, these cells were unable to form colonies, demonstrating that CDC42 and BEM1 are essential genes in C. albicans. The cdc42 null mutants failed to produce buds and hyphae and grew as large, round cells instead, suggesting that they lacked the ability to produce polarized cell growth. However, the cells still responded to hyphal inducing signals by aggregating and expressing hypha-specific genes, behaviours typical of the mycelial growth form of C. albicans. Budding cells and germ tubes of bem1 null mutants exhibited morphological abnormalities, demonstrating that BEM1 is essential for normal growth of both yeast and hyphae. Inducible, FLP-mediated gene deletion provides a powerful approach to generate conditional lethal C. albicans mutants and allows the functional analysis of essential genes.