Algal growth assays are the most frequently used methods to detect herbicide toxicity in environmental samples; however, these require several days to detect reductions in growth rate with adequate precision. Hence, a need exists for more rapid assays. Two in vivo chlorophyll a fluorescence assays, one using pulse-amplitude modulation (PAM) fluorescence and another based on fluorescence at 684 and 735 nm, detect the effects of photosystem (PS) II inhibitors (atrazine, diuron, and isoproturon) on Selenastrum capricornutum Printz only after 1 h and 30 min, respectively, of incubation. The median growth inhibition (IC50) could be predicted reliably from effects on three PAM parameters—maximal PSII quantum yield (ϕm), operational quantum yield (ϕ'm), and nonphotochemical quenching—and from measurements of fluorescence at 684 and 735 nm. The effects of the PSI inhibitor paraquat dichloride, were smaller in magnitude and could be detected after a 24-h incubation. These two in vivo chlorophyll a fluorescence assays can thus provide reliable, rapid, and cost-effective tools to screen toxicity caused by PSII inhibitors. Neither of the two fluorescence assays could consistently predict the effects of nonphotosynthetic inhibitors (alachlor, metsulfuron methyl, and diclofop methyl).