For the pathogenic yeast Candida albicans, secreted aspartyl proteinase (Sap) activity has been correlated with virulence. A family consisting of at least eight SAP genes can be drawn upon to produce Sap enzymatic activity. In this study, the levels of Sap1, Sap2, and Sap3 isoenzymes were monitored under a variety of growth conditions for several strains, including strain WO-1, which alternates between two switch phenotypes, white (W) and opaque (O). When cultured under proteinase-inducing conditions, most strains and W cells produce Sap2, while O cells produce Sap1, Sap2, and Sap3. Both W and O cells of strain WO-1 produce Saps in enriched and defined media that do not induce Saps from other strains. The specific Sap isoenzyme that is produced is determined by the cell type, while the level of Sap production is determined by environmental factors. The levels and temporal regulation of the SAP mRNAs as determined by Northern (RNA) analysis were consistent with Sap protein levels and with previous results. S1 analysis showed that SAP6 is the predominant SAP gene transcribed during hyphal induction at neutral pH. These studies define the culture conditions which control the levels of SAP mRNAs and Sap proteins, and they indicate that both the yeast/hyphal transition and phenotypic switching can determine which of the Sap isoenzymes is produced.