Conazole fungicides are commonly used to prevent fungal growth on turf grass and agricultural crops. As many of these sites are adjacent to coastal waterways and estuaries, there exists the potential for non-target effects of runoff on marine organisms. This study reports 96 h EC₅₀ values for four conazole fungicides (triadimefon = 5.98 mg/L; triadimenol = 5.51 mg/L; propiconazole = 2.33 mg/L; hexaconazole = 0.91 mg/L) to the alga, Dunaliella tertiolecta. We further investigated possible mechanisms of toxicity by examining sublethal effects of exposure on cell morphology, osmoregulatory function and lipid composition. These mechanistic studies revealed that conazole exposure does not inhibit synthesis of the cell's glycerol osmolyte, but does result in an overall increase in cellular volume and total lipid content. Both fungi and chlorophytes rely on ergosterol to maintain membrane structure and fluidity, and we provide evidence that the sterol-inhibiting conazoles may interfere with ergosterol biosynthesis in the cell membrane of Dunaliella. Employing elevated lipid content as a toxicity biomarker to triadimenol, an EC₅₀ of 2.25 mg/L was established, which is less than half the value reported (5.51 mg/L) when using population growth as an endpoint. These findings suggest that green algae may be especially susceptible to non-target effects of sterol-inhibiting fungicides in marine systems, and that suborganismal measures of pollutant effects should be considered when setting regulatory limits.