Candida glabrata switches spontaneously, reversibly and at high frequency among the following four phenotypes distinguishable by graded colony colouration on CuSO(4)-containing agar: white (Wh), light brown (LB), dark brown (DB) and very dark brown (vDB). These phenotypes also differ in a graded fashion in the level of expression of the metallothionein gene MTII (Wh<LB<DB>vDB), the frequency of switching (Wh>LB>DB>vDB) and colouration on phloxine B-containing agar (Wh>LB>DB>vDB). Switching among the four graded phenotypes is referred to as 'the core switching system'. An additional switch phenotype, 'irregular wrinkle' (IWr), has been identified, which exhibits a highly wrinkled colony morphology. The characteristics of IWr suggest that switching to and from this phenotype represents a second high-frequency switching system. A microscopic analysis revealed that during the first 3 days of colony development, cells in the centres of Wh, LB, DB and vDB colonies expressed almost exclusively the budding yeast phenotype. After 3 days, however, pseudohyphae and cells extending tubes accumulated, so that by 7 days the proportions of these two cellular phenotypes reached 40-50% and 10-20%, respectively. In contrast, IWr colonies were composed almost exclusively of pseudohyphae through the first 6 days of colony development. After 6 days, IWr colonies began to accumulate both budding yeast cells and tubes. The tubes formed by C. glabrata reached lengths of up to six cell diameters, but the tubes did not represent traditional compartmentalized hyphae. Tube growth ended when the tube tip expanded to form a bud. Tubes then functioned as corridors for daughter nucleus migration to the apical bud, and were ultimately left uncompartmentalized and nucleus free. Core switching, pseudohypha formation and tube formation occurred in a majority of 62 tested clinical isolates, demonstrating that these developmental programmes are general characteristics of most strains of C. glabrata.